Now I know what you are thinking "Darkstirling, that isn't true, Eldar was extremely strong at release!"
And that is absolutely true. However. when I say worst, I do not necessarily mean the weakest in absolute terms, I mean the weakest in that the internal faction balance of eldar is a mess.
-Most of their vehicles are garbage, but the falcon is great
-Most of their infantry are garbage, but dire avengers are great
-Wraithblades are still quite useless, but wraithguard are great
-Eldritch storm is completely useless
-3/4 of their doctrines are useless
See what I am trying to get at? Every faction has balance issues, but the Eldar are a faction of total high and lows. They have a roster of weak units that are carried along by the strong units. This results in decent game balance overall, but it feels bad to play Eldar, when I feel the need to chastise myself for wanting to build different units or try an exotic doctrine
Overall, it feels like eldar has far more useless doctrines, units and strategies than the other races. They simply rely on the strong ones much more. When I watch "pro eldar" play, they always build and do the exact same stuff every game. This is a problem for every race, but I notice it much more with eldar.