I have been trying some of the immutable linux OSes because from what I understand they are more modern and feature better security and reliability. What I have found so far is shocking. Half of these don’t support my laptop (probably because it’s nvidia optimus). Some I tried like guix were very difficult to install, configure, and use with sprase documentation. Good luck trying to use KDE, wayland, or pipewire for example. BlendOS was notably better and could at least run on my laptop but chocked with nvidia driver issues.
I have switched to pop os on my laptop for now but looking at alternatives and what to install on my desktop.
Compared to guix NixOS is significantly ahead. “difficult” is to each their own. For me, NixOS is the easiest to do certain things. But those things are what i use a computer for and might not match what you do.
As for docs - nothing beats Arch :)