Okay you try to use it twice in a berserk window alongside Upheaval as it adds up to 550 potency, stronger than Fell cleave(and all ogcd!), but that's only if you have enough rage when entering zerk. You want 3 Fell cleaves and 2 onslaughts/1 upheaval, but if you dont have enough, use 2 FCs, replace the third with onslaught x2/upheaval. Same goes for Inner release windows. Outside of berserk it's somewhat situational if you use FC but you usually get enough gauge for 2x onslaught/upheaval before Berserk starts again(and you should have almost 100 before entering Zerk too, 80 or 90).

So in other words, a lot.

Edit: Listen to Shinkyo^