It just melts everything really. Any half decent psyker with a flame staff will usually top the dmg chart by a large margin. Its less about individual enemy dmg then the extreme cleave, plus no ammo so you can just blast endlessly, plus the great stagger so your rarely ever in danger.
If my group of friends and/or associates I play with are having a bad day and having trouble clearing a Havoc 30+ mission, eventually one of them will get frustrated enough and say, “hold on, let me swap to my flame psyker” and then they’ll immediately clear the mission.
Straight up, it’s just OP as hell. As many others have said, for a long long time, it needs rebalanced. With the way psyser works I do not think you can really get rid of the no ammo/endless flame on without messing the class up. So I would think if they adjusted the dmg ticks, pulled back that near infinite cleave, and perhaps shift how well it works on heavy elites (crusher, bulwark, mauler) then it will stop being a “answer to every problem" weapon. Which is its core issue.
Just my 2 cents