One thing I found is that Cau Tau are annoying to fight in any game outside of the OG Tabletop. Deatchwatch pnp? A squad of fire warriors can be more threatening to your kill-team than most kind of enemies (unless you get them in melee OFC). Dawn of War? If you let a Tau player build up masses of fire teams, you're boned. Rogue Trader? Somehow the Tau player that was added shortly before the pnp was terminated can get a crisis suit by himself. I'm sure there are more examples you guys can think of.
When I learned they sacrificed the squats for these literal weaboos, my dislike over them grew immensely.
Oh, and as for the Adeptus Mechanicus not innovating, as mentioned they do, even if it's kept under wraps. The tech they hoard is ridiculously strong. The Mechanicus game has them using canon weapons that can obliterate necrons like cannons that create gravity distortions that crush armor, literal sunlight beams, advanced plasma weaponry, etc. They would be more than a match for the Tau and anyone else except the Necrons at their best, alas they don't share their tech because they can't reproduce it anymore/because grimderp.