I understand that he hates the film because of its many problems. Initially, Fincher had a wonderful vision but he wasn't allowed to implement it. Additionally, he was reportedly fired from the set multiple times. It's just that there are a lot of reports on the internet about Alien 3 that say:
"This film is so bad, even the director hates it!" and that's just stupid and shows bad reporting by idiots
Even if the film didn't turn out the way Fincher would have liked, at least the assembly cut is still a great film with a dark mood and a cold hopelessness, a brilliant soundtrack, and a cool alien and set design