I thought Andy Garcia, Eli Wallach, and Mantegna were worse than Sofia because they're actors and they stunk it up as much as she did. Adam Vinatieri was awful as well.

The dialogue was unbelievably bad.

As for the incest, I can't remember the specifics in Coppola's commentary, but his comment about that story and its source made it seem like it with better execution it wouldn't have been as bad as it ended up being. The commentary also made me appreciate the film's concept on paper. I keep laughing at how the other characters handled the incest by saying "it's dangerous" rather than saying straight up that it's fuckin' gross and all kinds of wrong.

Though Michael's confession is one of the best scenes in the entire trilogy as are parts of Michael and Kays scenes together. Also, "Light a candle for the archbishop" is a great line.