I'm level 37 and finding it's not quite as effective in nightmare as normal, but find it still helps a lot.
It's a whole lot of fun to have your pets holding the enemies at the doorway, walk up and get a 5 stack of soul harvest and cast out the swarm. Then you can see it spread through the rest of the room. On normal it used to kill the whole room, but now it leaves them at 20-50% HP in nightmare.
While I think it might not be the most effective skill in terms of damage, it's a lot of fun.