I've been thinking about this and playing Soldat (mostly deathmach) for a while. And I notivced something.
In Deathmatch, almost everyone uses the M79 and almost no one uses the minigun. It was not hard to figure why. It has 3 really big downsides to be considered a good PVP weapon:
1. The minigun has a long spin-up time, so when a enemy jumps at you from the edge of the screen, by the time it starts shooting you're already dead. Your reaction time doesn't matter, it not a weapon with which you can shoot back the instant you spot a threat. Makes sense ...sort off. Technicely, you could make a minigun that has a separate spin/fire control, so you can keep it spinning at full speed while you're in a danger zone (thus being able to start shooting instantly at full RoF), but that would give your position away rather loudly.
2. TTK, or Time To Kill. Simply put, assuming excellent accuracy it takes quite a while to kill someone with a minigun. 2-3 seconds of shooting at someone (and thus, being vulnerable to an attack from someone else) to actually kill him compared to instant kill from a M79? The spread of bullets is rather large and the damage small.
3. Realod time. It takes long to reload. Makes sense.
If you add to that the fact that it's uncontrolable in realistic mode and harder to master because of the push, then it's no wonder it's rarely used.