They go so smoothly that I always think I must have forgotten something. The only things I've learned to watch out for are networking changes, like the semi-recent interface name changes, network shares (samba protocol changes), and updates on stuff relating to encryption standards (ssh/ssl.)
I avoided that by accident because I migrated to bareos and used bareos package repo.
But in those case I think problems are plainly that the upstream doesn't really want to support distro packaging, we had some mess because the just one version decided... to start changing the name of packages and services for no good reason.
Most of the "bad script complained on uninstall" I got from 3rd party packages, which is funny as we have people in this thread complaining its "too hard" to get the package in the distro and the exact reason for that is to have as little problems like that as possible.
They go so smoothly that I always think I must have forgotten something. The only things I've learned to watch out for are networking changes, like the semi-recent interface name changes, network shares (samba protocol changes), and updates on stuff relating to encryption standards (ssh/ssl.)