Not a complain about rebalance, Dusknoir is good right now. However I think Dusknoir should be able to learn Shadow Ball TM. Its dusknoirs´ signature move and he should be able to learn it.
Metang has the ability Soundproof, which is supposed to reduce damage from "sound" type attacks. However, the move Sing from clefairies does put the sleep effect on him. I dont know if its supposed to be like that or if it is a bug.