In my experience it kind of just works. Takes a bit more maintenance than arch, around an hour a month at most. I had to reinstall it once and it was way easier than my arch, since way more stuff is in text files I can commit.

Also, since it's very very easy to rollback to a previous version, managing unpredictable issues is easy. I have a colleague that lost lots of time to an arch kernel panic during an update that required a reinstall. On NixOS I can reboot, choose the last derivation, work and fix that if/when I want.