I back up the entirety of my /home directory except for cache, temp, trash (trash is stored at /home/$user/.local/share/Trash), Download folder, and a folder I named "NOBACKUP".
It backs up a lot of stuff I probably don't need, but I'd rather back up more than I need, than to be caught not backing up something that I did need.
edit: oh, I have a btrfs snapshot of /root too, but I don't think that's something the backup tool in Ubuntu can do since it defaults to ext4
I tried to give Windows 11 another go recently just to see how it is, I pulled all my files over including my gog games files which had wineprefixes in the folders, with /appdata folders for each prefix.
Windows decided "you know what, screw c:\users\appdata, lets use the appdata folder in this random gamefolder on a different drive instead" and proceeded to cannibalize itself just breaking the majority of apps. No idea how it can't recognize that the random wine "windows" files that aren't in the correct locations aren't the actual location for them. Couldn't fix it because it thought the "c:" folder in the wine directory was my actual c: drive and refused to delete it
Sure it was an extremely niche issue a Windows user would never realistically run into, but it reminded me just how fragile it is for uncommon usecases