Hopefully I'm preaching to the choir and the main response from everybody is that this is obvious and didn't need to be said.
I read a few comments from some gross cishets about how angry they are about the female Custode. They prop up the Imperium of Man as though they are some paragon of morality and not the picture of everything wrong with imperialism. Naturally this perfectly moral force would have only men in its ranks, clearly! They don't seem to realize that if women are left out we have a sexist/mysogynist system which is not worth praising, and if they include women then it's the imperialist machine trying to keep up appearances. Frankly, I've always figured the Imperium didn't care about gender or race because everyone is suitable to die in combat and be ground into food for the starving populace.
tl;dr Men are upset that women exist and missed the whole satire
The Tau are hopebright and good and everything bad about this is Imperial propaganda and the Smurf players can just cope and seethe about it. For the greater good!
The grimdark part of the Tau empire is that despite being very morally good, they'd still be completely wiped out if any of the other factions considered them a threat.
Also, while this is less confirmed, it's also implied that the Tau's belief in The Greater Good could eventually wind up creating a new warp god leading to the destruction of their civilization. The tau are the equivalent of a squadron of fresh recruits surrounded by entire armies of people who hate them. That's why they still fit in the grimdark universe. They're the plucky, good civilization with no real shot of surviving.
They're practically nulls, how the heck would that work?
Yeah, but the humans and other races they seek to integrate into their society aren't. They'd either have to do some necron-tier shit and destroy the souls of their citizens or they'd be at risk of humans creating a new chaos god.
Another option is they may decide that warp-sensitive races are too much of a risk to the galaxy and adopt a genocidal attitude for the greater good. That would be thematically appropriate for Warhammer 40k. A bad encounter with the effects of the warp leading to them abandoning their previous principles. Kinda like how the Horus Heresy affected the Imperium of Man.
What kind of chaos god would they make? I wouldn't be terribly opposed to a lord of altruism or comradery, though I don't know too much about the Tau.
You fundamentally misunderstand the Tau. They're not altruistic, they're just the least-worst civilization. They're closer to a space Roman Empire than they are to communism. They'll conquer you, put you in a pseudo-slave caste, and collect taxes. They can only claim to be a lesser evil because every other civilization's default response is genocide.
Probably something to do with absolute control. The Greater Good is an authoritarian philosophy in which individual citizens don't really have any rights. They have certain cultural freedoms of course, but no right to self-determination. For warp-sensitive races that believe in this philosophy, it would show up as a desire to dissolve the ego into the whole. This could create a god which seeks to create a galaxy wide hive-mind of sorts. A sort of warp based Tyranid empire composed of multiple races which would subject entire planets to experimentation for the greater good.
Ah yeah, that makes more sense. Fits the setting much better.