> updates are fully atomic and every change can be rolled back by booting into an old generation

Well, the updates themselves yeah, but not what data they use. You cannot always rollback database upgrades for example, without also having to rollback the data source of the database. In most cases you're right though.

I'm saying this as someone who is a fan of NixOS and use it on all my servers because I tend to forget what I do if I just ssh in and fix stuff. Although I'm on Arch/CachyOS on all other hardware.