I'm not an expert, but I don't buy "The Godfather" himself. In reality, their personal lives and careers were proabably closer to Al Capone, or the many of Latino gang leaders sitting in prison today.
Yeah, there's your problem. First off, they were
nothing like Al Capone, who was never an East Coast mobster. And furthermore, Al Capone, albeit not the most sophisticated mobster in the world, was incredibly cunning and ruthless.
This is about second/third generation Italian immigrant mafiosa, and it's pretty good in that respect. Yes, they speak Italian, most East Coast-based mafias did, because they had to communicate back and forth to Sicily. Also, Vito Corleone is based on actual mobster, Frank Costello, who was born in Italy, carried some of the same mannerisms and whose life parallels Vito Corleone. I think Puzo has said that he's also based on Vito Genovese (gasp - also born in Italy!), who tried to put a hit on Costello and failed, leading to his early retirement from the mafia.
Point being, the mafia was like that. They were cunning and ruthless and nobody did see it coming, and it was until the 70s that the authorities were finally able to break the mafia down and that was mostly due to inner turmoil.