During design, we have tried to make them competitive, i.e. not underpowered. The problem with dark elves is, that their expected properties (elite, few, fragile) make them weak in Mordheim. Some of their typical tactics (flanking, fleeing charge) is not possible in Mordheim. They are limited to 12 models, have no access to strength skills, have T limited to 3, and are expensive. Their default ranged weapon is, if you compare price to effect ratio, inferior to longbow, and maybe even to bow. (BTW, in this comparison, the best weapon is sling.) Their higher M sometimes helps, but skaven, lizards, posessed, and anyone on horse are still faster and ability to charge first does not help that much, if you cannot wound or stay the countercharge. They start with BS 4, which makes them almost as good at shooting as Reiklanders (who have longer range and more shooters). WS 4 or 5 makes them hit on 3, but it does not protect - any ordinary WS 3 model will still hit them on 4. High I can be helpful, but high S and T is usually better. High Ld is nice, but most warbands have ways to compensate for it (hiring bard, various Ld reroll skills etc.) So they got some bonuses, like cheap poisons or RXB discount at the beginning.
In our group, when it gets to overpowered warbands, we usually talk about dwarfs (you have to roll 6 to get them out of action, and they are rich enough to compensate for their low numbers) and skaven (fast, cheap, numerous, good skills and equipment, and low Ld does not matter if you don't have to roll it).