USKP v1.3.1 - (2013-03-18): "Decapitations with the devastating blow perk should not be taking place with blunt weapons. It should now only happen with battle axes and greatswords, as would be expected. (Bug #91)"
I get why the change was made, but I also think that someone could totally get through someones neck with a giant hammer. Especially since many do have blades and spikes on them too.