Greatsowrds have far better range and can be wielded with both two hands and one, tests have been run where greatsords have lopped off three heads in a row, in one swing. Mind you, this is a standard with greatswords. Scimitars have a more aerodynamic design, but you can hardly slice an opponent with no head.
Though it may be true that a great sword is quite slow and requires strength to wield, it is only a problem that it is slow when used by an incompetent person. In fact slowness can add greater ease in precision, and its weight makes it a meat cleaver to human bodies.