Depends how you define 'success'.
Purists will say it means winning a trophy; some would say Champions League qualification; some would say any European qualification.
For most clubs, these are not realistic definitions. The question is, whether Everton 2002 were one of those clubs. Let's be honest, we were.
One realistic measure of success would be, say, achieving better than other teams with comparable budgets; another, for the manager, might be to do better than the managers at the same club who preceded and succeeded you. On both of these measures, his time here was a success. Fergie might subliminally have had ulterior motives, but he was canny and he rated the job Moyes did here so highly that he personally recommended him.
For a team who had been regularly fighting relegation and who were outspent by every team that finished above us and some that didn't, an average 7th place finish, including 4th one year, was a massive improvement from where he found us. We got close to a couple of trophies because he took a poor team and made them better.
I share your frustration that he clearly had his limits, which seemed to cost us those chances of a trophy when they were there. It was galling. The football wasn't always pretty, but it got better over time. There were some poor signings, but some excellent ones for very little money: Coleman, Pienaar, Baines, Cahill, Arteta, Stones, Jags, Lescott, Distin, Yobo. Any one of these would be first name on the team sheet now.
On balance, it's hard to say his first stint with us wasn't successful by any reasonable measure. I'm not convinced his 2nd term here will be as positive tbh. I agree there are better managers out there, but who knows if they would have come? I hope we're not paying him £5m a year and I hope he isn't still manager in 3 years unless we've qualified for Europe or won a trophy. Most importantly, though, I don't want the club relegated. Secondly, it would be nice to see a few goals, or at least shots on target. If nothing improves, it's going to be pretty tough, like, but I think he'll get more out of them than Dyche. Surely...?