I think NB's acting went downhill in S6-7. As I understand it, that's when his personal problems also began affecting his work. I think the writers had to deal with that and the fans' complaints are unfair in that regard.

Agreed on ITW. I can sort of write that off because Xander plausibly believed it even if it wasn't true. But his self-righteousness generally grated on me, and his attempts at suave came across badly too. He did loyal and supportive pretty well when called upon.