I would separate NixOS from other immutable distros. NixOS is really about giving you blank slate and letting you fully configure it.
You do that configuration using a static config language that is able to be far more idempotent than Andible. It’s also able to define packages that are well contained and don’t require dynamic linking setup by manually installing other packages.
Immutable distros, on the other hand, really have no advantage to your setup and will probably feel more restrictive. The main use I see for them is for someone new or lazy that wants to get a working system up and running quickly.
My favorite example of how idempotent NixOS is has to do with the DE. If you've ever looked at switching from gnome to KDE, or the other way around, most distros suggest to just re-install because each DE leaves so much cruft around and it's so hard to remove everything in a safe manner.
With NixOS, you just change one line in your config, and the DE is cleanly swapped.