That being said, there is still room for Tau in 40k.
When Tau were first introduced, there were hints and suggestions that this polished, high scifi race of gundam pilots wasn't as noble as they looked. If you scratched a little, a pattern starts to emerge around the concept of "greater good". Almost every evil man in modern history has evoked the "greater good" as his motive for evil, whether it be Hitler, Pol Pot, Mao Zedong, Stalin, or any other tyrant. Were they lying? Maybe.
The Tau embody this concept. They have the potential to be the North Korea of 40k, with their shining, wealthy, and peaceful cities full of diverse peoples all getting along in harmony are just shams erected to seduce the desperate. They are the propaganda town with windows painted onto the walls of their buildings, singing out a message of light that could be yours, too, if only you joined them.
The Communist theme runs deeper when you look at the Ethereals, who appear out of nowhere in Tau history and almost magically convince the entire species to lay down their arms and submit to their will. Their influence is hypnotic, much like Marxism continues to seduce people with its message even after its countless atrocities. The Ethereals are hiding something, and they have the entire Tau species as their unwitting pawns in some secret game they are playing. The character Farsight lends weight to this narrative, as the commander's escape from the Ethereal's influence causes him to defect from the empire (the border guard who jumped the fence).
If these aspects of the lore were brought to the forefront and used to characterize Tau, everything else about them would work. It seems like this was the writers' original intention for them, with hints about the sterilization of undesirable races and such scattered throughout the lore. Unfortunately, GW has taken the race a different way since their inception and painted them as shallow protagonists.