I don't really want to get into semantics around the words "very good" because it's a frankly boring discussion, but needless to say he's had very good stints at Everton, West Ham and even Preston, and had them achieving good outcomes relative to resources over sustained periods. I don't believe it's all about trophies because there are only 4 to win and with the way football is they are going to be heavily biased towards big clubs.
He's had failures and notably appeared to fail at the next level so I'm not saying he's a top manager, but I'd say the sum total of his career would be that he's a good manager, very good, however you want to say it that does not upset your sensibilities.