I am mostly familiar with the Tau through the video games, the novels and binging TVTropes (as I don't have the time or the money to get into the tabletop scene), and in my opinion they are actually under-utilized. There are so many great storytelling opportunities they would provide to contrast them in the silly grimdark universe, but most writers just don't seem to really care about them and instead want them to be grimdark too for some weird conformity logic.
In fact the reason I love the Ciaphas Cain novels with all my heart to this day is because they actually gave us glimpses of the non-grimdark, non-warring, everyday life of the WH40k universe, where the foot-soldiers are actually human beings and the ones in charge are trying to do their best to save civilians instead of treating them like numbers on a spreadsheet and where battles have their stakes and reasons. These kinds of things gave life to the Imperium of Man, which I previously considered the silliest, most forced grimdark thing in existence, and the Tau was a great opportunity to do the same to the entire setting, seeing the universe through the eyes of a species that has yet to be jaded out of their skull by millenia of pointless xenophobia and warfare... In fact, in many ways they would provide a very nice thematic throwback to pre-Horus Heresy humanity, as the ambitious, rapidly advancing, science-focused species of the setting.
But no, we have to focus on the combat specs, give them grimdark undertones to fit in the setting better and hate them for not conforming, because that is how you keep a franchise fresh and interesting, right?