Going back to the original question, I absolutely loathe the Tau aesthetic.
Outside of their suits, they just seem like really boring, generic aliens.
As for their actual suits, the best way I can describe them is that they look like transformers toys that don't actually transform. You know how transformers toys often looked weird, because they needed to have parts that folded in/out or vehicle-parts that they couldn't quite hide? That's the vibe I get from tau suits.
I mean, look at the battlesuit in the Coalition Command and tell me it doesn't look like it's supposed to transform into some sort of weird jetfighter:
https://www.games-workshop.com/en-GB/Tau-Empire-Coalition-Command
Anyway, I know that this is entirely subjective, but this sort of aesthetic just doesn't appeal to me. Probably doesn't help that I've never been a fan of mecha-type stuff.
In terms of gameplay, they can definitely be rather one-dimensional. However, I think their least fun thing was Jump-Shoot-Jump. Shooting stuff and then jumping behind
LoS-blocking terrain to prevent most/all return fire did not make for a fun game. Thankfully, this has largely been removed in 8th, but I imagine many people still associate Tau with this.
Another possibility as to why people dislike Tau is that a young, naive race idolising Communism might hit a little too close to home for some.