The Hateful Abiliity isn't that much an issue balance-wise regarding sheer damage-output, but rather in that it pretty much streamlines towards one specific playstyle - high ini/ambush, free action declare as enemy, then asap, maximize. Then, 10 rounds no mayor ability for that one. Think of it as a micro-nova. It's one of the most efficient combos, but one that somewhat decreased the fun-factor in my tests. My players are all for power-gaming, but after 2 battles, the conclusion was that they'd not take the combo again, since it simply wasn't that fun for them.
I did think about it and will probably make a variant (if you don't do it) that instead provides a smaller either constant damage boost or a smaller boost that has an individual cool-down. That way, the intent of the feat and its synergies are maintained, but the micro-nova-issue goes away.