You have to consider whole body of work. Benitez best in a Valencia for me, and early Liverpool. Since then he has been mediocre, but his reputation is from his early high point in his career. He got the Real job really because that's were he started and Perez wants a local hardworking type after the messy strong star personality coaches.
Mancini all things considered have some won league titles and several league cups, you aren't being fair to him you don't acknowledge his core achievements in his work. He is limited if you compare him to top coaches, but you are comparing him to ones who haven't even shown anything in top top level. Many upstart coaches have gotten chances for top teams and weren't good enough, or never did get a chance because they didn't show enough.
I am spending much of a discussion on him.As I pointed out he won that titles when we were in a knockdown after coming from Serie B coached by DelNeri/Ferrara/Ranieri.Secco was our sporting director.So I can't praise him being a good coach winning several titles in that environment.When he had a chance to shine if he had potential at City and have whatever player he wants he failed,just that mentioned title,luckily won.After that he went to Gala in order to continue coaching.After Gala he was out of job and was offering his services to us every now and then publicly.I don't compare him to coaches who never won anything at top clubs since Ventura never coach a top club and Montella was coach at Roma shortly as his first job and only Capello achieved a title there .Also Roma ,having in mind their history isn't a top club.Same goes to Fiorentina.