Except for my home directory, I have a little bit state on my machine, ssh host keys, WiFi credentials, journal logs (I almost want to remove this, but I bet that would bite me in the ass one day)
The root partition is a tmpfs ram disk, and everything not saved in a separate partition, and either mounted or symlinked on to my root partition is lost at shutdown and regenerated at boot. Its less than a second extra to boot, and it uses about 16MB extra memory. Everything is made very convenient through the impermanence nix module and the sops nix module
Just a reminder that nix packages works on most any distro, and then they work like other universal packages, but without the sandboxing
NixOS is just when you take this to the logical conclusion and have every part of the distro packaged in Nix, including configuring your OS, and optionally your users configuration files as well.