As has been said, Wesker might just seem less strong than the others (for me at least) because of the nature of the game being 3v3, meaning Wesker has a comparatively lesser effect on the match.
And for that same reason it's also less ridiculous to me to see 6 Weskers in the final 8 than say like 3 Yuns in the final 8, I'd say. Teams are bigger so just because Wesker is on a lot of teams doesn't mean he's carrying most of them.
But I also feel that with some slight nerfs (removing the damage buff he gets when he removes his glasses after one of his ultras or just lowering his health a bit) Wesker simply falls into the top tier, rather than "probably best in the game but not broken." Wesker has a decent amount of bad match-ups and while his gun assist is a good tool it doesn't really control a match like better assists can. Actually Wesker has a lot of bad match-ups, kinda, it's just that he's good against a lot of the meta-game and his bad matches aren't that bad cause he's kinda versatile. I'd say most small characters or characters with a good range game have an advantage over him though.
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I tried to have a fierce rivalry with SuperNiceDog, but he made me look worse than a GameFAQs Contest!