The main advantage of the minigun over other styles of support machineguns, is that you get to divide the heat generated by sustained rapid fire between multiple barrels, as opposed to heating only one barrel. This means the gun can fire at greater rates and for longer periods of time before the barrels start melting.
The tradeoff includes more moving parts and electronic systems in the firing mechanism, meaning there's just that much more that can go wrong with it. It also weighs a lot more, because of those extra barrels you're lugging around, the external power source and the ton of ammo you'll be putting through it. The motor also needs to be wound up before you start shooting, taking a moment and making a lot of noise, which may pose a problem in tactical situations.
The problem with squad support machineguns tends to be that they are too heavy, fire too quickly and have too much recoil. They are meant to provide heavy cover fire and create kill zones, not for area denial. Also, with guns in general, you want to have as few moving or electric parts as possible, because that means fewer things can go wrong with them, and they're easier to fix.
Multi-barreled weapons are great when you need to suffuse a large area with sustained fire, and have lots of spare weight for them and an engineer on hand to make sure they keep firing. It was a good move to put them on helicopters and armoured vehicles - and Australian developer Metal Storm has made one hell of a scary fixed-position area denial drone - but perhaps not so much to assign them to infantry squads.
For man-portable automatic support firepower, I suggest you look at this:http://world.guns.ru/machine/mg71-e.htm
Still heavy, but much more managably so, and with far less recoil. Plus, it converts into a handy grenade launcher, for when you really need to blow stuff up.