The Tau are interesting. Originally, they were much more unambiguously "good" (or at least naive), being the only faction to actually work with other species. Hell, the Tau were multiple subspecies, with the air caste actually having wings. Contrary to their reputation as communists nowadays, the inspiration behind them was NATO troops, specifically things like the multi-species coalition, caring about their troops at all, focusing on light mechanized infantry, and using lots of euphemisms for terms for military action/organization (as we see in the "police action" in korea, for instance). The imperium meets with them and don't understand how their society functions without an extremely oppressive state to keep people in line and assume it must be ethereal pheromones, but this take is quite obviously meant to be imperial propaganda.

This is then slowly retconned, and now you have canon ethereal mind control. The four subspecies? Oh, they're now an oppressive caste system. Imperium players love the Farsight Enclaves because they're free of the influence of the evil CCP Ethereals, completely ignoring that Farsight is possessed by a demon. The multi-species coalition part of the model line has been completely ignored in favor of moar battlesuits; kroot never got models after the range launch (I guess there's the kill team now at least), vespid were a one-time thing, and we'll never get models of the psychic polar bears. How much of this was individual GW author brainworms and how much was conscious change after the initial rage at Tau's release, who knows. And of course, you can't actually have them progress in a meaningful way despite centuries having passed, because the setting can't change (even when it does).