How is he a failure though? He was a success at both Udinese and Roma.
As for an embarrasment in europe, again, how was a pre juve allegri better than him in europe? He never did anything special with a milan team that was, at least in his first two years, much more superior in terms of personnel to Spalettis Roma.
You obviously dont like his style and do not want juve playing that style, which is fair enough. But in terms of accomplishments, i think its hard to argue that a pre juve allegri was any better than Spaletti.
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Spaletti did quite well in the league with a roma side that was a mess before he came, it was almost impossible for him to compete with an inter team that had much more resources. He reached the CL quarters just like allegri did with Milan. The Milan squad that Allegri inherited had Pirlo, a peak Zlatan and Thiago Silva. Spalletti never had such players at his disposal. I really think there is very little between them.