The problem, for me at least, is in the map with hallways, if a fusion rifle catches you in a hallway no matter how close you are or what gun you have you are dead. Other than that I think they're fine.
Hallways are to Fusion Rifles what clear sight lines are to Sniper Rifles; it's where they're strongest. You could try the sliding shotgun trick, but generally you should frag them and/or run to where you have the advantage.
Not much room for counter play really.