people complain about xin being simple and oppressive when strong

Oh they are hated as well. Don't you notice how people want them nerfed as well?

Just because they're doing fine doesn't mean we should buff others like them. Pretty sure that's a logical fallacy.

And people complain about Annie. Not so much Pantheon but to be fair, he tends to be used as a counter pick so an inflated win rate would not be unnatural.

Pantheon is a pain to play against, it's just the fact that people see him less and are thus less inclined to complain on the boards about him.

Annie though, I'm actually ok with, especially now that her stun is more telegraphed. I can see why people would hate her though.

I have never seen Xin complaints so I dunno

And I don't know where your stats come from, but both Annie and Panth were below 53% in champion.gg