I use it to host one of my self-hosting servers at Hetzner, and I’ve really enjoyed the experience. Especially having the ability to roll back when i mess up is a life saver.
In my experience NixOS is really stable which makes it perfect for running on a server.
I use NixOS, which is kind of a middle thing. The OS is generally immutable, except through one central config. This allows to tweak the OS to exactly the way you want it, whilst preventing any accidental changes and allowing atomic rollbacks.
Learning Nix configuration syntax is a bit of a bump in the road, but once you’ve got that it’s smooth sailing