Well, there's practically only one team in Italy, and that's Juventus. Rafa might not be able to topple them, but to not even come in the top 3 places in that league - without the likes of Inter and AC providing a challenge - is pretty poor. Factor in the fact that Napoli have more money than pretty much the entire league, bar Juventus, and finishing fifth looks even worse.
Nobody can say Benitez isn't a good manager in a one off game, and I'd have him above almost anyone if our season was decided in one game. Therefore it's no surprise he can win a one off game like the Italian supercup. However, it's over the course of a season that Rafa keeps falling short. 5th in the serie A, and being knocked out of the Champions League, and subsequently the Europa League by Dnipro (Europe being his supposed strong point), point to his power waning.
The fact that he's won one cup and a glorified pre-season friendly against Juventus don't look good in isolation. Even when comparing these "achievements" to our recent turgidness they still appear meh. Comparing them to how we've fared is more indicative of how bad we are than how good he is.