Added to the discussion:

The Eldar Void Spinner


As GlynG mentioned this is being tried at a discounted price (dropping to 650 from 750pts), so this comparison might help inform the discussion as to whether the Triple Void Spinner (3VS) formation requires a price drop.
I'm repeating the first post with the new points added in Biel Tan green for easy comparing.

Ordinatus Medrengard 600pts
WE 10cm Armour 5+ CC 6+ FF 4+
Siege Cannon 60cm 9BP, Disrupt, Indirect Fire
2x Mars Bolters 30cm AP5+/AA6+
2x Lascannon 45cm AT5+
DC 4: 4 Void Shields, RA.
Critical Hit: The Ordinatus’ plasma reactor explodes in a roiling blast of energy, the war engine is destroyed and all units within 15cm suffer a hit on a 4+

Steel Legion / Minervans
Artillery Company 650 points
Any nine of the following units:
Basilisk, Manticore

[] Basilisk
AV 20cm Armour 5+ CC 6+ FF5+
Earthshaker 120cm AP4+/AT4+ or 120cm 1BP, Ind

[] Manticore
AV 20cm Armour 6+ CC 6+ FF5+
Rocket Launcher
150cm 2BP, D, Ind, Slow firing

Baran Siegemasters
Artillery Company 425 points
Nine Götterdämmerung Howitzers and nine Gun Emplacements

[] Götterdämmerung Howitzer
LV 0cm Armour - CC - FF 6+ Howitzer 90cm 1BP, Ind

Death Korps
Artillery Support Company 600 points
Nine Earthshaker Platforms and nine of the following:
Gun Emplacements (5+ Cover Save) or Trojans

[] Earthshaker Platform
LV 0cm Armour 6+ 6+ 6+
Earthshaker 120cm AP4+/AT4+ or 120cm 1BP, Ind

Eldar: Biel Tan
3 Void Spinner Formation
WE 25cm Armour 5+ CC6+ FF5+
Void Spinner Array 60cm 3BP, Disrupt, Ind
DC3, Skimmer, Reinforced Armour. Critical Hit Effect: The unit is destroyed. All units within 5cm suffer a hit on a roll of 6+.


-------------------------------------------------

Damage Output:
All of the above can put out 9BP per turn .
Triple Void Spinner: also 9BP per turn
Steel Legion and Minervan Artillery Companies can also be specified to use slow firing Manticores to fire 10 one turn and 8 the next, for the same 9BP average per turn.

Result: All do the same average damage per turn.


Blast Markers
Kreig Earthshakers and Baran Siegemasters do not Disrupt; they will cause 2 BM on the target plus BM from units lost.
SL and Minervan Basilisks also don't have Disrupt. If using Manticores instead they will gain Disrupt, generally producing a significant amount of extra BM on top of the initial 2.
Ordinatus Medrengard has disrupt, generally producing a significant amount of extra BM.

Result: The Ordinatus is significantly better at causing BM than Krieg and Baran artillery companies, and significantly better than SL and Minervan artillery companies with Basilisks. If SL and Minervans use Manticores instead they come out slightly ahead (by 1 BM) every second turn.
Triple Void Spinner: Equal to Ordinatus, meaning considerably better than the others


Resilience
9 AP or AT hits are required to kill 9 Howitzers (light vehicles)
10.5 AT hits are required to kill 9 Manticores
12 AP or AT hits are required to kill 9 Earthshaker Platforms or Howitzers (light vehicles) that are in Gun Emplacements (5+ Cover)
A little over 10 AT hits are required to kill 1 Ordinatus (4 to remove Void Shields, about 6.24 to get through DC4 5+RA)
12 AT hits are required to kill 9 Basilisks

Result: The light vehicle artillery batteries are almost always going to be significantly easier to kill due to taking hits from both AP and AT weapons, and because AP weapons generally hit on lower numbers than AT weapons.
The 9 Basilisk formation (Armour 5+) require the most AT hits to kill. The Ordinatus requires about 2 less AT hits to kill, although the actual amount required may increase because the Void Shields recharge (two VS recharges would make it require about as many hits as Basilisks).
Triple Void Spinner: Requires 14 AT hits; Winner


Suppression
Theoretically:
The Ordinatus requires 4 BM to break
The Artillery formations require 9 BM to break (exception: Krieg, which can take additional LV to boost numbers but have a whole different set of AP vulnerabilities)
Triple Void Spinner: Requires 9 BM to break

In reality:
If the Ordinatus takes 4 hits, it will be broken and lose its void shields and be broken, but take no damage.
If the Artillery Companies take 4 hits,
Basilisks would be reduced by about 3, leaving 6 survivors with 4 BM and requiring more one more hit to break.
(Note: while not broken, this formation now only has 2 models that can fire)
Manticores would be reduced by about 3.3, leaving 5.7 survivors with 3 or 4 BM and requiring more one more hit to break.
(Note: while not broken, this formation now only has 1 or 2 models that can fire)

Result: The Ordinatus is easier to suppress, but not by nearly as much as might be expected when looking at the relative size of formations.
The other formations are reduced in number as well as being suppressed.

Triple Void Spinner: If it takes 4 hits, it might lose 1 member (2.1 damage), but it is more likely to lose a member only after 5 hits (2.8 damage). Once one is destroyed, two more BM would be required to break the formation, but one member would already be suppressed so firepower would drop from 9BP to 3BP.
Result: The 3VS is as hard to suppress as the large artillery formations, and its fire power is degraded at a slower rate because it has better armour, because 3 BM are required to suppress each model and because (bar Criticals) it only losing a model after every 3rd damage point. 3VS is clearly harder to suppress than Artillery Formations.
It is harder to suppress than the Ordinatus, but its firepower is degraded at a faster rate.

Regrouping
The Ordinatus regroups on a 3+ and fires at full effect (9BP Disrupt)1
A broken Basilisk formation that's been broken once may have around 5 or 6 survivors, 1 or 2 of whom will be suppressed (assuming Commissar) meaning 3BP or 4BP of firepower.
Result: A massive advantage to the Ordinatus
Triple Void Spinner: The 3VS regroups on a 4+, and will generally have lost 1 member and have 1 remaining member suppressed meaning it regroups with 3BP of firepower.


Initiative
Ordinatus 1+, all others 2+
Result: Advantage to the Ordinatus; Initiative is particularly important to indirect firing units kept out of Line of Fire.
Triple Void Spinner: 3VS initiative 2+


Resilience to Counter Battery Artillery fire
In the type of artillery duel common between two armies with large artillery assets:
The Ordinatus can take a maximum of 2 hits (average 1 at AT4+) from an Artillery battery that centres the template on it. It would take 2BM from basilisks, or 3BM from Manticores firing just over half their slow-fire weapons. It is likely to lose 1 Void Shield, maximum one.
The three templates fired are likely to cover about 8 members of an Artillery formation if it is spaced out well, or 9 and part of another unit if it is not. If 8 are covered, it is likely to take 2+4=6BM and lose 3 models (closer to 4 if Manticores or Howitzers), meaning that it is a very common result that the formation breaks. If unbroken, the most common result would be that only 1 gun is still available to fire (1 or 2BP)
Result: Overwhelming advantage to the Ordinatus; no artillery formation can hope to win a duel with a disrupt-artillery WE.

Triple Void Spinner: The 3VS can take a maximum of 4 hits (average 2 at AT4+) from an Artillery Battery that centres a template on one and manages to clip the other two (note that DC3 WE can be spaced widely apart to prevent this).
This is likely to cause 1 damage but has only a small chance of causing a kill. The Ordinatus would on average cause 4 BM due to Disrupt, suppressing 1 VS and reducing it to 6BP. The non-disrupt artillery formations would cause 3BP, also supressing 1 VS and reducing firepower to 6BP.


Critical Hits and TK weapons
None of the formation being reviewed can suffer additional damage from an Critical result, or take multiple hits from a TK weapon.
Result: Disadvantage for the Ordinatus

Triple Void Spinner: The 3VS can take additional damage from Crits and TKs. They are initially worse vs TKs and Crits than Ordinatus because they have no Void Shields, and later better because Crits are restricted to killing one third of their unit rather than the whole.

Range
Basilisk 120/240 cm
Manticore 150/300 cm
Howitzer 90/180 cm
Ordinatus 60/120 cm
Void Spinner 60/120cm
Note that the range of 120cm means that there are small areas at the edge of the opponent's deployment zone that the Ordinatus cannot reach even if placed centrally and on the 15cm line of its own deployment zone.
Result: Clear advantage to the artillery that can set up anywhere and still reach anything on the board. Disadvantage to the Ordinatus and the Void Spinner


FF Assault
The first 4 hits on the Ordinatus would be Shields, and not count for combat resolution. Once the Shield is down, 5+RA stops just over half (5/9) of the hits. It has 4 attacks at FF 4+ (2 hits).
Hits on other artillery formations generally result in kills (66% of hits on Basilisks, 83% on Manticores, 100% on Howitzers) which count towards combat resolution. They all have 9 attacks at FF6+ (1.5 hits). They should have +1 from a commissar (assuming assigned to this formation and not a casualty).
Result: Advantage to the Ordinatus
The 3VS do a total of 9 FF5+ (average 3 hits). They do not have void shields to mitigate the first FF hits, but 5+RA stops almost half the hits.
They are far more likely to outnumber their enemy than the Ordinatus, and slightly more likely than the Artillery Formations (due to taking less FF Casualties) but have no commissar.


CC Assault
CC bypasses Void Shields, so hit would be against the unit's hull which count towards assault resolution. 5+RA stops just over half (5/9) of the hits. It has 4 attacks at FF 6+ (.66 hits).
Hits on other artillery formations generally result in kills (66% of hits on Basilisks, 83% on Manticores, 100% on Howitzers) which count towards combat resolution. They all have 9 attacks at CC6+ (1.5 hits), except for howitzers which are CC-. They should have +1 from a commissar (assuming assigned to this formation and not a casualty).
Result: The Ordinatus should be similar to Basilisks; The basilisks stop less hits so have more casualties for resolution, but are a larger formation and have a commissar.
The clear losers are howitzers with CC-
Void Spinner: Winner. The 3VS can force CC to become FF, a big advantage over the other artillery formations.


Air Assault
The Ordinatus has 2x AA6+
None of the other Artillery formations being reviewed have anti-air included in their cost. If taken as they come:
Result: Advantage to the Ordinatus. With two 6+ shots the chance to shoot enemy aircraft down is small, but it is guaranteed to place at least one BM on an unit doing an air assault to potentially gain 'I have no BM' and 'You have more BM' bonuses to combat resolution.
OR
SL and Minervas can add 1 or 2 hydras respectively, for 50pts each.
Result: This would make them better against air assault (Hydras are 45cm 2xAA5+ while the Ordinatus' native AA is 30cm 2xAA6+). This makes the price difference larger, but means the the category is a win to Minervans, followed by SL.
The Krieg and Baran Siegemasters artillery formations don't have access to Hydras so remain considerably worse vs Air Assault.

Void Spinner: Does not have AA.

Other Weapons:
Manticores and Basilisks have Heavy Bolters: 1x AP6+
Baran Howitzers have no secondary weapons
Krieg Earthshakers have no secondary weapons
The Ordinatus has 2x Mars Bolters 30cm AP5+/AA6+ and 2x Lascannon 45cm AT5+
Void Spinner: No secondary weapons

Price:
Steel Legion / Minervans Artillery Company 650pts
Death Korps 600pts
Ordinatus Medrengard 600pts
Baran Siegemasters Artillery Company 425 points
Void Spinner: 750/650 points
Result: Clear advantage to the Baran Seigemasters, who pay considerably less than the other forces being compared.

(If anyone thinks any factors are missing or spots any mistakes please let me know, thanks)

Players almost never take Arty companies/groups of three void spinners though; 2 formations of 3 Manticores are almost always better than 1 x 9 formation of Manticores and are cheaper to boot. Same holds for Void Spinners.

Iron Warriors can't split their Ordinatus into smaller formations though.

In a game where any 600+ pt formation is almost certainly going to be your BTS formation, something as squishy as an Arty company or a DC4 5+RA formation is always going to be a big risk.

Good write up Matt.

Personally thought the unit was ok, just needed a different approach than say a reaver or other similarly priced WE. The disrupt ability is a little scary but then if your mech inf just stay in your transports and you drastically reduce its chance to cause BM.

I think the best thing about the unit is the way it complements the rest of the IW list by almost forcing you to close where the defilers and SM formation can shoot or tear you apart.

It would be interesting to see it with a Fixed Fwd arc give it is:
A: a WE and most WE have a limit on their weapon arcs unless in a turret.
B: its a massive siege gun an presumably needs to set itself up before it fires etc.

This might help improve its public opinion without having to reduce its firepower or stats. The only negative is that the IW player will need to think hard about the objectives and game during deployment so the cannon is setup in the right position to support. If not they may need to consider redeploying which will obviously drastically reduce its capability. Theme wise this works for me, I certainly don't want to see its firepower toned down, its is what it is.

Very thorough write up Matt - the conclusion being that the ordinatus is better than a lot of sub-par formations that are rarely taken.
In my opinion the most important factor is how it works in the list, as it stands it definitely would be far from an automatic choicefor me
- no cheap formations to protect it, to protect it I'm going to have to spend about 1000pts

- vulnerability, with no fearless and 10cm move lose an assault and its pretty much auto killed as it can't get away. Get broken and its dead too. Your opponent knows its going to be on the blitz so against anybody with a spaceship, deathstrikes or out ranging artillery your in trouble

- eldar I take void spinners as eldar have poor saves and small units so are very vulnerable to barrages+ to target AA for air assaulting. IG as I've got lots of no/poor save infantry which are vulnerable to barrages. Marines or CSM I'm not that worried aboyt anything but MW barrages. Particularly for IW I'm going to want to get close for assaults and short range shooting. As such the Ordinatus is a luxury as I'd prefer to spend the points on more assault/assault support formations or, if I'm going to buy artillery on artillery batteries to lay BMs.

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Thanks for the extra work you've put in Matt!
Definitely food for thought.

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I said I'd put up some opinions/recommendations following all the analysis. These are based on the stats and comparisons I posted but shouldn't be treated as more than just my own views.

The Guard Artillery formations:
All far too fragile, hard to regroup, vulnerable to Blast Markers and any form of assault to be credible as competitive BTS choices. They should be assessed for points decreases (circa 75-100 for all except Baran who would be around 50pts) or should continue to be ignored by most players not building an army for a "siege theme".
That's not a new point of view, so I imagine the reason it's never been implemented is because the end result still wouldn't be a compelling enough choice to make pushing for it worth while

Void Spinner formation:
I posted my view here in the Eldar thread viewtopic.php?p=512293#p512293 but in a nutshell the Void Spinner one of the best units in the entire game, but it is a unit so good that it is a reason to take a specific army. I recommend leaving it at its current price rather than decreasing it, or even making it 275 to make a multiple unit formation more appealing rather than reducing the cost of the multiple formation. The latter idea is not likely to gain much traction among eldar players, so just leaving the cost as it is instead of dropping it to 200 for additional Void Spinners might be simpler.

Ordinatus Medrengard:
There are several ways this could go since I know the list champion Steve would like it to be Iconic
Iconic as ubiquitous = In which case I'd recommend dropping Disrupt and the cost to around 500 to make it a very appealing unit for an excellent price
Iconic as Feared = In which case I'd recommend improving it slightly in the area it didn't beat everything else (damage capacity) to DC5 VS3 (actually a larger improvement over DC4 VS4) and improve the AA to 2xAA5+ to make a price bump to around 750 more palatable to IW players while the damage output 9BP-Disrupt becomes more acceptable to opponents because of the increased cost.
Or Iconic as the IW Signature unit, which it currently is, in which case it's probably worth a bit more than the current cost (625-650) because of how simply awesome it is in this type of army, or same price (600) minus its AA ability.
It sounds like Steve's considering a small price bump for an unchanged unit, which sounds like a decent compromise to me.