I think there's something interesting in the idea that the object is mystical and that's why she's more suited for wielding it.

My only question is why would it being mystical make it prefer Slayers? Trolls are hunted by slayers (I assume since the way that Buffy got it was because of hunting Olaf) and vampires are mystical too. Since trolls think babies make delectable meals, I don't see troll hammers having some sort of moral issue causing it to disfavor vampires in favor slayers because Slayers are better people (generally speaking).