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
I’ve migrated in a few days, and figured out the basics after a week or so. I’ve been using Linux on-and-off for about 11 years, but I don’t think that’s a requirement for NixOS :p
I think the most killer features are the reproducability and the rollbacks directly from the bootloader to a previous generation. So even if your new system configuration won’t boot anymore, you’ll still be able to revert back without any external tools.