It amazes me how many people complain about Tau and Eldar. They're not *that* good. Strong, sure, but not game-breakingly unbeatable by a long shot. I've fought Eldar five times with Deathwing and/or Drop Wolves, and performed five wins, two of which were tablings. Tau, I've only fought twice but I got a tabling and then a loss on First Blood. Tau and Eldar both have insane issues with fragility that are very easy to take advantage of.
Now try it with Sisters.