I bought my first Mac, an iMac G4, in 2002. It came installed with Mac OS X 10.1 Puma, and I've been installing minor and major Mac OS X and macOS updates ever since, with the one exception of Mac OS X 10.7 Lion, which in my opinion was the worst Mac OS version ever (before now). My current machine is a 2014 MacBook Pro, which I had updated from Mavericks through Mojave. I did skip macOS 10.15 Catalina, however, which I've heard was very buggy, and I had been holding off as long as possible on macOS 11 Big Sur, which I've seen is very ugly. Ultimately, though, I couldn't wait any longer, because I need to run the latest Xcode beta in order to develop Safari extensions for iOS, and the latest Xcode beta requires Big Sur, so I finally decided to update to Big Sur on Saturday. This was a disaster.
Over the years I've seen many Mac bugs, and even some kernel panics, but I've never had a Mac OS update just fail to finish. What happened this time? I started with the latest version of macOS Mojave on my 2014 MacBook Pro, and I had plenty of free disk space for the update, more than 100 GB. I don't think I had any third-party kernel extensions installed. In fact, I uninstalled Little Snitch before updating, because I was still using an older version of Little Snitch with a legacy kernel extension, which isn't supported on Big Sur. The only thing "unusual" about my Mojave volume is that it had FileVault enabled.