One recent example was me trying out KDE Plasma. Usually I use AwesomeVM but the HN thread about the new Plasma version made me try it out in Arch. After the install and trying to use it for a while, I wanted to go back.
Uninstalling the package(s) is not enough, as it already has been overwriting bunch of stuff and changed configs that are not being rolled back after uninstalling.
Still to this day KRunner launches (via some d-bus command or something like that) when I do my Super+R shortcut, which usually runs the AwesomeVM launcher, not KRunner. Haven't figured out how to get rid of it yet, but got other stuff to do right now, so simply living with it for now.
With NixOS, I'd just reboot and select the previous version where KDE Plasma was never installed.
Another fun related feature: NixOS really only requires /nix and /boot to boot. So, some of us just nuke everything except /nix, /boot, and /home on boot:
The problem was: in the AUR, lots of packages didn't built or launch, and even in the official repos, some packages were missing libraries or just not having the right versions.
I was tired of tinkering to get everything to work as I wanted.
I used Arch for a while before NixOS. Another problem I encountered with the AUR is that, since the AUR is not built as a single consistent system with Arch itself, often packages from AUR would start failing because some library was upgraded in Arch and the newer version was ABI-incompatible with the version that the AUR package was compiled with.
last major issue I had was when everything moved from /bin to /usr/bin.