Heavy's candidacy for uber is highly dependent on the situation. If there's a pyro ready to airblast you, he's likely going to waste most of your time if not all of it, which is where soldiers and demomen are better uber candidates. Heavy is also not always the best choice if you're trying to take down a sentry, although it sometimes works. If you can catch people around a blind corner, though, or you're in a fire-fight in a somewhat close quarter situation, heavy uber can be horrifying. Same goes for kritzkrieg.