Meh that's why I don't like the Tau, lf. Little buggers got off so easy. Let's examine shall we?
Humans - Live in just about the worst dictatorship ever invented, under the hand of a semi dead man kept alive on the sacrifices of his subjects. And he will NEVER DIE so they can't really hope for regime change. Mostly inhabit festering hive worlds swiss cheesed with tunnels and full of cities built on the ruins of cities over and over until they spear high into the deadly poisonous ruined atmosphere. Not to mention the fact that just about all the Chaos gods have it in for them and get the majority of their forces through corrupting humans and twisting them to do their dark bidding. And all their technology is many thousands of years old. Completely stagnant society. The only thing keeping it going is the momentum of 40,000 years.
Eldar - The last gasp of a race that by all rights died out 10,000 years ago when their own hedonistic lifestyle gave birth to their very own personal Chaos god. Stuck living on craftworlds floating aimlessly through the galaxy with no real home worlds to call their own. Death can't even release them from their fate, because when they die Slaanesh is waiting in the warp to consume their soul. Instead their souls get transferred to a tape backup and kept in stasis just hanging around for all eternity on the same old craftworld. But at least they've got style.
Orks - Are pretty well off, being too dumb to have any tragic history. But still... they're dumb. All they know is fighting and killing.
Chaos - Slaves to dark gods, need I say more?
Tyranids - I don't know, I actually like these guys. Sure, they descend on whole worlds and strip them of all natural resources until there's nothing left but dead and airless husks of rock... but at least they do that pretty well. There's something very pure about them. You know where you stand with them, eh?
Squats - AHAHAHAHAHAHAHAHAHAHAHAHAHAH ... breathe ... HAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAA ... HAH!
The Necrontyr - Whoops, we made a deal with ancient beings that get off on consuming whole systems and find the terror and pain of their inhabitants makes a nice piquant seasoning. Now we're stuck in bodies of metal for all eternity fighting their battles for them.
AND THE TAU --- Happy, clean, nice. Their technology is new and it's advancing. Have no warp signature. When they die, there's no evil force of Chaos waiting to consume their soul. Don't even have the potential of birthing a Chaos god, because of the no warp signature thing. Would have been wiped out by the Imperium for their improper and unblessed technology millenia ago, but got saved by a convenient warp storm surrounding them for aeons. Instead of enslaving races, they go around befriending them. UGH!
We can only hope that the next invading Tyranid hive fleet happens to break into the galaxy right where the Tau worlds are and shows these little blue bastards what the 41st millenium is like.