It's a shame that the central premise of the movie doesn't work because the movie around it (the Vatican/Mafia ties, Altobello, Lucchesi) are all interesting things. The weakest point of the film is actually Michael himself IMO both his central story and the way he is portrayed. We don't get to see WHY or how he changed back to being more like his pre 1945 self.

We're left with an embittered and alone Michael Corleone in 1968 at the end of II, and then in 1979 we have this talkative, nice guy who actually has a personality that is more a mix of Vito and an older Fredo than his old self. He's weak like Fredo (in that he is too foolish to see the threats around him and wishes to make peace with his enemies), overly ruled by his emotions like Sonny (IE letting his personal disdain for Zasa cloud his judgement) and timid like the older, sickly Vito was. It's almost like he had a stroke and his personality changed. Not even a semblance of the quiet, contemplative, almost genius level intelligence is there. Michael in II is like a master chessplayer whereas III he's like...A guy? Some weak mix of the worst of Vito, Sonny and Fredo? Too mired in the small details to see the big schemes around him?