Well, they don't shoot particularly well, have to be taken in lumps of 10+ which drives their price up, pay a big chunk for their mandatory gun upgrade and then can't shoot if they're in a vehicle.
So, ultimately, Dire Avenger squads end up doing it better.
But yeah, in 5th mech is king. Mech isn't the only option, but with vehicles being wonderful metal boxes of survivability, you need them or something that can compete. Eldar, as an aging codex, gets one decent build: put your dudes in bawkses, and run around dropping s6 firepower on stuff and generally neutralizing the other side's battle plan until the end of the game, where you use your superior mobility to claim objectives. For killpoints we just try to not lose a whole lot of men.
Her(?) name may be slightly inapropriate, but she just needs love! And I just need to eat your face!
I'm probably going to follow my usual Eldar tactic of driving around the table "circling and screaming 'I don't give a fuck' with my windows down and system up" and avoiding anything that looks scary.