Here's an old thread on the subject, where Michele (one of the rules developers) providers some thoughts.
I like it when a game provides a formal framework/guidance/limits on this kind of spending, but can understand why the designers went a flexible route here.
Personally, I tend to limit things like Dreadful Spells or Strike Fear to once per round, but I'd be happy to spend multiple points on something like Snake Like Speed. So I'm generally "Each Fell Ability can have hate spent on it once per round" but with notable exceptions.
Burying the PCs with Fell abilities doesn't make sense narratively and nor does it make for a good mechanical challenge. "The Barrow wight appears and chucks 'Dreadful Spells' six times, burning its hate down to 0" isn't exactly kosher.