The main thing that's missing about a Great Maul is how the weapon actually worked, which is true for most weapons when they're converted to RPGs (so no biggee)
Basically, the reason to not use a Great Maul is it's slooooooowwww. Axes are hard to ready after you swing with them, swords less so. But swinging, essentially, a sledgehammer, is not quick. It leaves you off balance and open to attack.
However, even worse than a cutting implement like a sword, greatsword, what have you, even when you hit armor or a shield, you seriously dent/bash/injure/knock the hell over that which is behind the armor or shield. The sheer weight of a maul head *moves* that which it hits. A well placed maul shot to the legs is going to crush said legs no matter what kind of plate armor is covering them. A well placed maul shot to a shield is going to end up practically knocking someone over and denting the shield if not outright bending it in half, with a good chance of shattering someone's forearm.
Just think about sledgehammers, now, and how and why they are used instead of long blades.
