look what he did with Charlton and look how they fell after he left. He became West Ham manager 2 seasons ago but resigned not long after because he wasnt being given control over who the club signed. West Ham were doing quite well. That was his only job since leaving Charlton and he had been at Charlton for years before that.
I definitely wouldnt put Curbishly is the same bracket as someone like Megson. He has proven himself to be a good manager unlike Megson who gets the sack from every club he goes to.