Although I don't like seeing people speak poorly of all of metal, as if it could be shoved into one tiny little hole and treated as bazillions of copies of the same thing, the impression I got from the Opeth commentary was nothing like that. As with anything else, 99% of metal is worthless garbage and there's no point in denying that. Metal doesn't have to be 'limiting', of course (unless you buy into the worst combination of elitism and narrow-mindedness imaginable), but those that find metal to be freeing and not limiting are, sadly, not very common.
I don't think metal can outdo jazz as far as covering ground - at best, they're at about the same level there.
Jeff