Perhaps an indication of whether he did well or is whether each club improved upon his departure. Liverpool became even worse, as did Inter Milan. Both clubs have/had deep rooted problems within the hierarchy of the club, which compromised their chances of success more than any manager could.
He finished 3rd with Chelsea with a Europa League. As an interim coach, his objective was to stabilize the club and ensure CL football - which he did. They proceeded to then finish in the exact same league position the year after with Mourinho, suggesting that Benitez' performance was pretty solid.
At Napoli, he won the Coppa Italia in his first season and finished 3rd. It was pretty improbable that Napoli would be able to break the stranglehold that Juventus have on Serie A, but again - Benitez brought silverware to the club. He was fairly popular with Napoli fans at this stage.
His second season didn't go as well and they lost out on CL football. They've improved since and currently sit second in Serie A.
He left Real four points behind Atletico Madrid, who were leaders at the time. They'd also cruised through the CL group stages. Four months later, they sit in the same position - suggesting that he had Madrid performing on par with where they should be.
Contrary to your claims, I'd say his record suggests that he's a very solid manager. Just unfortunate that he's become a bit of a parody to the general footballing contemporary.