I think we're starting to lose sight of what this topic is about here...
Can someone use a heavy effectively? Sure, I guess so. But when compared to other body types, the heavy is pointless: the heavy weapons are lackluster, slower movement speed is infuriating, inability to mantle/reach certain places or objects forces you into death-trap choke points, and the health bonus is negligible. What this mean is that someone could use a medium/light equally as effective as a heavy, meaning that there is no point in choosing it.
Saying that "Just because YOU can't do the body type justice, doesn't mean it's unbalanced. Just means you're not as good with it as someone who can." is a fallacy. A medium can fill exactly the same role as a heavy on the front lines. If both a heavy and a medium take ARs to the thick of battle they will have the exact same combat effectiveness, if not leaning more towards medium because of the increased mobility. If a heavy takes an HMG they are literally handicapping themselves because, from 50 hours of experience, I can tell you that the heavy weapons' (which do, on average, 6 damage less than the ARs) increased ammo count does not outweigh the increased recoil.
I can guarantee that if you played a game as a light in the same fashion as a heavy, you will do just as good if not better than if you played it as an actual heavy.
If I take a Medium body, and try to fight the way I do as a Heavy, I guarantee you I would be slaughtered. When I do it as a Heavy, it works.
People who are good as Heavies CAN'T do the same with a Medium.
What I said isn't a fallacy, you're misinterpreting it completely.
"Slow movement speed is infuriating" FOR YOU. But NOT for someone who can adapt to it effectively. The health bonus seems "negligible" TO YOU, because it isn't huge, and with the way YOU try playing a Heavy, you'd need it to be. That doesn't match my experience when going up against someone who's good at playing as a Heavy, and I'm far from being the only one to support their value in the game.
Just because you've put 50 hours into the game, doesn't mean you're good at playing AS A HEAVY. Even if you've spent 50 hours as a Heavy, you could still svck at it. My girlfriend's brother has played about 100 hours combined of Assassin's Creed 2 and Brotherhood and it's still painful to see how bad he is at them.
You can stop presenting one-sided evidence based on personal experience and claiming it's a valid counter to someone else's personal experience which proves you WRONG now.
EDIT: If I can boil your next argument down to "But it doesn't work when I do it, so you must be lying," I'm to edit your quote to more clearly state your point. K? Good.