For what it is worth, the recent remaster of Gf3, Gf3: Coda improves the storytelling of Gf3 significantly and I would say makes the movie considerably better, but it is still not competitive with Gf1 and Gf2. That said, I really recommend everyone watch the Coda remaster at this point and do it with an open mind.
While I'll agree Sofia is a much better director than actress (she’s a truly outstanding writer and director) and that Ryder would have been better (Coppola later did work with her on Bram Stoker's Dracula), I’m not going to be mad at him casting his daughter or even blaming it on nepotism. After all, all the previous examples of nepotism, like working with his father, his sister, his brother-in-law, his nephew, etc worked out quite well. Like, the whole Coppola family thing is that they all work together and hire each other. She wasn’t a great actress, but I don’t think she’s want sunk that film. The truth is, any film in a series coming 15 years after two of the greatest films of all time (and the rare sequel that is arguably better than the original, which was already a masterpiece) would have an impossible standard to live up to. Even if Godfather III was a perfect film, I’m not sure it could live up to the hype that the first two carried, especially after 15 years.
And it wasn’t a perfect film. It’s not a piece of garbage but it isn’t in the same league as the other two, which again, are two of the absolute greatest films of all time. But seeing how successful nepotism has been for the Coppola family, I think blaming his teenage daughter is fairly weak.
TL; DR: it was a hard first role for a young actress
I don't mind the Godfather III. It's not nearly the movie its predecessors were but it's also not nearly as bad as some claim, but during any scene with Coppola in it, I literally need to skip forward to avoid the sheer cringe effect she creates with her dialogue, facial expressions and gestures..
Should someone have spotted this and replaced her? Maybe, but her scenes were not crucial enough for anyone to care to tell the director.
ever read the book? It's a dog.
It's worth reading to appreciate the many modifications that transformed it from a mediocre novel into a brilliant movie.