TL;DR: All linear fusions except sleeper should get a slight Nerf and move to the energy slot. I believe linear fusion rifles need to be moved to the energy slot, or the need a damage buff to put them into closer ranger to that of Sleeper simulant. My reasoning for this is pretty simple. Linear fusion rifles are a hybrid sniper/fusion. They function much like a sniper in that they are made for precision. They do comparable damage to a sniper, or perhaps a bit more. Yet, they are a heavy weapon. Why? I feel like they got left in that slot with the Forsaken updates while shotguns fusions and snipers became special. The only ones that stayed behind in the heavy slot were the ones capable of extreme damage output that would obviously far outclass all other weapons of the same type. That or they would need to be nerfed, which would cause outrage and make them far less special. So, Legend of Acrius, Tractor Cannon, D.A.R.C.I., and Whisper stay heavy. But why do linear fusions stay behind? I feel like what plagued shotguns, fusions, and snipers in year 1 now plagues linear fusions. The exotic weapons in each class so obviously outclass the legendary weapons that no one would really choose to use those instead. The only real reason you wouldn't use the exotic was if you wanted to use a different one in another slot. Why would I ever choose any linear fusion but sleeper? Why, only if there's a different exotic I want to use in another slot. Nerfing sleeper isn't the answer because it would no longer feel special, like an actual power weapon. So the better answer is, slightly Nerf all other linear fusions and move them to the energy slot.
The reasons I like them in the heavy slot is that they make for a good "all purpose" heavy and I can still carry a regular fusion. Initially I was like you, "why are Linear Fusions still heavy??" I said. However, with their RoF and mag size buffs, they make a formidable boss burner while still holding enough ammo to take down a pesky Hobgoblin or two. On the one hand I'd like them to be Energy weapons, but on the other hand I'm glad they are still heavy and carry the power that they do.