There are a few units that tend to disappoint.
For Dark Eldar, it has been their vehicles, so easily shot down. This is apparently going to change. Also most of the models are poor, and grotesques and mandrakes are just not very good at anything.... Dark Eldar also have a number of the WORST models citadel ever put out: (1) Incubi. (2) Hellions (3) Scourges (4) Warp Beasts (5) Dark Eldar warriors (6) Wyches. These are the top worst six citadel minis ALL IN ONE ARMY. And they have a codex dating from 1998, which is before some players were even born! All in all this is the list in need of some work. It is a great concept, and could become a really great list when they finally re-do it. For now it is the land of disappointment.
For Tyrannids, the latest Lictors and Ravenors are a little weaker. Also how do Biovores end up getting to shoot three spore mines EACH per turn? It used to be an ordanance template, which was good, but small template and shootable mines make them a fairly weedy option when you could be loading up 200 points of Carnifex instead, or three Zoantrophes all with psychic scream!
For CSM the new codex has pulverised the 2002 lists. It is not that units are weak or overpriced, but it is sad that so many fun (and overpowered) things have run away to start their own Codex! Where have the Bloodthirsters gone?
Oh that's right! To Apocalypse! How sick!
If you want to nominate a cruddy unit, how about IG infantry, set in lines with their single heavy weapons waiting for the enemy charge! I'm not saying they lack power for points or anything like that, but what kind of tactical options are there with squads of guys who lose their one decent shot if they move? Nearly everything in the game can run and fight.
I think IG infantry lines are the worst units, especially when they are deployed and played as a defensive fire line1 YAWN!