The mini-gun has the accuracy of a laser pointer with no shot dispersion over the general aim area (which is quite unrealistic) and no area/shrapnel effect on impact. This renders it completely useless against infantry unless you are stationary or a phenomenal shot, the first of which compromises your safety, the latter being highly unlikely while whizzing by at 150 km/hr. It's akin to trying to shoot a man sized target, while flying, with a standard infantry rifle. In the helo showcase, I spent a good 15 minutes trying to kill a single man.
By comparison, the AH-6's shot dispersion is nearly perfect, giving you a nice balance between the skill needed for accuracy, and a realistic "grouping" of rounds.