Well, FFG might have to re-imagine the Eldar to make them playable especially if they were originally not intended to be so.
They don't have to. In the 40k universe, the stamp on humanity is "Sucks to be you!". FFG can just play this card and make it clear that being a human is not a good thing when your companions are people like the ancient Eldar. When done right, this could be actually even more interesting than a "balanced" race selection.
Chances are that the Eldar have some major flaw somewhere. It could be their underestimation of humanity by way of their ego or something.
I don't know where I've seen it originally, but in some fan-made expansion the Eldar has this Trait called Civilization Bound that gives him/her penalties when he/she is out in the wilderness. That is actually pretty cool.