Anti-Air gunsThe part that seems to get the most flak in the Squat list is the flak itself. (Bad pun intended). Squats come with no aircraft or spacecraft at all, which denies them a very mobile facet of the Epic gameplay. Secondly, the only decent AA gun they have is on a very fragile and most importantly
immobile chassis, the Thunderfire.

There is also the smaller 45cm 5+ AA flak cannon on the Overlord, so the army is not *completely* without AA, however there are enough issues (and guns!) on the overlord that it is a whole separate topic!
What balance issues have arisen with the way the Squat army handles AA?No other army has no aircraft and no mobile AA. There are a few AA pieces with zero move (Siegemasters, Death Korps etc) , but the vast majority of them have some sort of transport option. As a result, this leaves the Squats with the following fallout:
1) Squats are *VERY* susceptible to losing the air war. If the opponent has little or no aircraft, then the Squats do well. If the opponent is Aircraft Heavy, or they at least manage to knock out the Squat AA early, then the game can end up very one sided.
2) Overuse of Castling Strategy. With immobile AA the only serious choice, the Squat general is then forced to lean towards the concept of a static army build. Lots of immobile Goliath cannons and other static artillery that can hide inside the AA bubble and protect each other. This can be a very boring and frustrating build to play against.
3) The Thunderfire cannon is exceptionally shooty. Ignoring the AA portion for a moment, the base cannon is AP3+/AT3+ and quite long range. Considering that the 0cm move means that you are likely to issue
Sustained Fire orders that means you are hitting most AT on a 2+, which is a shot that is only matched by the exceptionally rare and expensive IG Vanquisher cannon.
3) The Thunderfire cannon is exceptionally Fragile.The TF cannon has paper thin armour and is bought in formations of two units. i can't think of any other formation in the game that is bought in such a small number. (War Machines don't count as they have multiple hits.) Sneeze at the uni hard enough and you will kill/break them immediately.
The upshot of all this is that that the thunderfire cannon rules as they stand lean heavily towards the bad reputation that the squats currently have:
A largely static army with overpowered cannons that either wins big, or is smashed apart easily. It's this static glass cannon brittleness that leaves a bad taste in a lot of players mouths, weather they be playing with or against the Space Dwarves.