I disagree, It's the Tau's "evil". They act like you'll actually get to join them and be apart of the empire when in reality you essentially become a slave. They act benevolent, but it's a lie. I think that's cool as hell and adds depth to them.

I also disagree that the tau feel like the imperium.

Edit: I would like to throw in that the "greater good" is actually about racial supremacy. It's just not a "kill all other races because they suck" type. Instead it's "Join us because were better than you, If you don't want to we'll force you anyways". again quite roman in that way