On Heavy? Yeah, they probably are overpowered in pubs. And I don't mean low-skill servers; I mean high population matches. More for the same reason Scout is overpowered in 1v1 and 2v2. Scout flourishes when movement is unrestricted; when they can run past one enemy and sneak a win; when they have lots of space to dodge. On the flip side, the counter to Heavy is "be where the Heavy isn't". But if it's a 16v16 pub and the enemy team has six Heavies? You can't be where the Heavy isn't.
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Cats land on their feet. Toast lands peanut butter side down. A cat with toast strapped to its back will hover above the ground in a state of quantum indecision