He was largely a failure at Milan - his interim stint with Chelsea was decent - he hasn't improved Napoli that much.
He didn't even get a chance at Inter, Mourinho left the squad in exactly the same shape that SAF left Man Utd in when Moyes took over and like him, Rafa wasn't even given a pot to [oops] in, what could he do. so he ended up leaving by mutual consent, what has Inter won since, nothing.
Napoli aren't the richest club in the world, he only had money to spend in his first season due to Cavani leaving, looking at the squad, Rafa has improved it no end. Performance wise, they are where they should be, in the top 3. He won the Copa Italia last season and finished 3rd. This season he's still competing in all trophies and they are currently 3rd, if he had money to spend, who knows where he would be.