Wade Phillips was a bad HC. He was a much better DC.
Williams could be a good one or a bad one. It's just too early to tell. He makes some rookie coaching mistakes, but he also seems to fix the mistakes he's made. The Bills have made improvements while he was the HC.
Our GM, Tom Donahoe was very impressed by him. Go look up some of the other coordinators Donahoe hired for HC jobs. You'll see that he has hired some good ones in the past. When it comes to picking out talent, Donahoe is one of the best.