Was it necessary to close Is a clean install better than upgrading? as opinion-based? The phrase "clean install" should be enough to suggest to an informed reviewer that a clean install sometimes has less problems than an upgraded operating system. A clean install is also quick and easy for new users to do. If you're looking for edge cases and corner cases you can find them in this question alongside good advice like backing up files before upgrading, but that's not sufficient reason to categorize the whole question as opinion-based. It seems to me that the edge cases and corner cases have dominated the good advice here.
This question's timeline shows that this question was already reopened once after being closed and I voted to reopen it at that time. Two of the five reviewers who close voted it this time are no longer users at Ask Ubuntu. I think it's time to reopen this question again.