It depends. If they have other abilities that are worth something, I’d go with game breaking.
Example: OG Captain America. With sig ability he can get 100% perfect block chance on a good matchup. Is it game breaking? No. If it was added to a kit like Dr Dooms (or more recently Domino) then maybe. But if they just added 100% block proficiency to meh champions like Daredevil, Taskmater, Loki and possibly silver surfer then they could be MUCH more viable in content. Not game breaking, just better.
It depends. If they have other abilities that are worth something, I’d go with game breaking. What if they didn’t? I’d think game breaking either way.