Hard role or not, first-time actress or not, and all nuance aside, she just acted terribly, full stop. Her acting was very nearly the definition of high school theater mediocrity and simply awful in virtually every scene. It would have looked bad in a completely average movie with completely average (but not bad) actors. But to watch her "act" in scenes with people like Pacino and in the case of this movie, especially Andy Garcia, it's just actoral travesty.
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..