I've noticed that the exotic perk on doom fist pauldron doesn't seem to proc on melee kills with a sword, even if you don't have power ammo for it (making it effectively a regular melee attack). I don't know if this is intentional or not, but it seems like that would be an odd design choice.
-
I don't believe that's a bug. It states "melee ability kills" which means your charged melee ability has to be the killing blow. Swords, ammo or not, would prevent your ability from working to my knowledge.