Mid-update you can end up with broken dependencies and thus not fully working system. There may also be a problem with how programs update their config files.
This is especially true with big updates which is why Arch and Manjaro recommended to update from plain TTY during the Plasma 6 upgrade. Reboot, do not login, switch to TTY, stop the graphical login manager, just then proceed with upgrade.
A bit too much to ask for from regular users. Especially since it relies on them reading update announcements before each update.
Personally, I always close most stuff during updates. I even had terminal emulator itself die during update on Linux Mint.