Just tested with frostmourne vs Mappo and Lich king, i borrowed the item and Unfortunately i didn't had enough time to test more than once cause my friend had to run a crucible event.
Well Mappo took a while to kill like 1 or 1.5 minute but Lk died in first 30 seconds, the boost that skeletal burst gained is just optimal, like ''kill fast revive go'' , i am really impressed so far thats just my first experience i tried butcher's as well and it was good enough.