Perhaps. Your call.
When human infantry fights human infantry, with halfway modern firearms, on the offensive they tend to split into different sub-units for fire and maneuver tactics. One sub-unit fires, to make the enemy take cover, the other sub-unit takes the opportunity to maneuver into a more advantageous position, then the roles switch. For an overview, look at chapter 3 of this US Army manual.
So there are three requirements for an infantry weapon.
- It has to do enough damage to affect a typical target. The speed of yóur grains takes care of that.
- It has to come with enough ammunition to keep up suppressive fire. You've taken care of that in your setting, too. In the real world, consider the switch from 7.62mm or .30 battle rifles to 5.56mm or .223 assault rifles and carbines.
- It has to have a sufficiently high rate of fire. Compare Napoleonic or American Civil War era infantry formations with the slaughter of WWI.
Put two weapons next to each other. One is the minigun, with a high rate of fire, near-infinite ammunition and damage, and the need to carry a backpack cooling system. The other is a single-barrel variant, also with near-infinite ammunition and damage, but with a somewhat lower rate of fire due to to the weaker cooling system.
How accurate are both weapons? If the grains move at close to lightspeed, do they have time to be affected by wind or whatever? It makes little difference if a target is hit by one grain or ten. Point against the minigun.
Or could the minigun deliberately incorporate a 'shotgun effect' where a burst spreads a little? Point for the minigun.