I would agree that miniguns aren't useful. In that respect, they are exactly how they are in life. Too cumbersome vs other options (shotgun, assault rifle, flamethrower, toxigun) to use in close quarters combat, too unwieldy vs the assault rifle to use in cover-to-cover close range skirmishing, insufficient damage vs rockets and machine guns for point defense, and too inaccurate and poor at range for anything else.
The only way I see it being useful is against loads of small, squishy targets bumrushing me eg. hybrids. However, that is never something that's a big deal anyway. Might as well pack some real guns eg. Magnum and just use lots of snap shots. The minigun actually reduces the amount of soldiers stat improvement because it can only do one shot per turn.
