Even though each races have their fair share of under power units. This post will focus on Eldar only, as I playe Eldar the most.
Wraithblade. For a tier 3 unit, they are especially under power. They are expensive yet they can not win against SM Dreadnought (A tier 2 unit). Sure, they are a little bit cheaper, but you have to consider the cost of tier up to get them. They can not tank enough damage nor they can deal any significant damage to your opponent. Not only that, they are a group of units (3 Wraithblade per squad) Which mean they will drop model, and it is very expensive to reinforce. A tank or a Dreadnought can be repair from very low HP. This make them much more cost effective.
Dire Avenger (DA). Hold your horse, I mean Dire Avenger in late game. Just like orc boyz, they are useless late game. Even then you can use orc boyz to stun (with doctrine) the enemy unit late game, and suicide them quite decently. DA is useless late game. No reason to spam them at all. Give them an upgrade for late game please.
Vyper. They are quite decent in harassment but will render useless very quickly. And SM land speeder is just better at everything. Not only that, you have to commit resources to build a sky portal, which in many case you would rather have infinity gate. SM can switch tech from land speeder to anything they want easily.
Warp Spider Squad. One of the worse Elite in game, no doubt. For a 5 point heroes, they are too week. They can't snipe enemy infantry as they get mold down so fast and their damage is abysmal. Their teleport is decent but it is very hard to use correctly. No one want to teleport an Eldar army into the enemy, because Eldar is range heavy and very fragile. And if you use their teleport to escape, the enemy can teleport with him and chaise him down easily. What is their purpose? They are not a tanker, nor a damage dealer, nor a good support unit. The only good thing about them is their minds, but even then, a good player will know how to counter minds easily.
That is some of my thought after more than hundreds of hours in this game. Please tell me what you guy think.