Umm well IMHO, as with many Eldar units, I suspect the view that they are 'overpowered' may only b true where they are used en-mass. Put another way it is very much down to how they are used.
Individually Eldar WE are generally poorer in comparison with IG WE in most departments.
- Weaker assault stats (FF5+ as compared to FF4+),
- Shorter range (60cm MW as compared to 90cm TK),
- Weaker armour (5+RA compared to 4+RA),
- More expensive (250 points compared to 200 points).
They are slightly faster by 5cm, have skimmer and of course they get hit-and-run all of which can be used to hide them from enemy fire.
Like most artillery formations down the ages, their power does tend to increase with numbers. So three Scorpions concentrating on a part of the battlefield have more than three times the effect of a single WE etc. The problem with this analysis is that we tend to play 3000 point games where players are hard put to get sufficient numbers for these effects to be demonstrated. For example 2x Guardians, 9x WE and an Aspect with the SC might look quite formidible on paper, but I doubt it would do very well against air assaults, massed artillery or Hoard armies like Tiny-Tim's speed freaks.