we dont know much about the Tau we know about there cast system and the concept of grater good. when we think of communism we think of no individual freedom and oppressive regimes. but if you took away those notions douse the codex say they are kept in line by secret police ?. imagine something different. every one has a voice. can say what they want. hold there own opinion. they have good homes, good food, free time to advance them selves personally through works of art. are never hungry. in return they do a job that provides to the society they live in and sustains that way of life.
the Trial by fire is different for each cast. but all are based on proving your self, you advance through action. not by just waiting for inevitable progression to happen. that why the tau have advanced so fast. each is driven to succeed and to better them self for them self and all those around him.
they would still deserve there place in the 40k universe even more so. the fact that on the scale of thing the odds are stacked against them is just as grim dark as them being a dictatorship or sadists