Let me put it like this: it’s about learning curve. Arch is relatively easy to begin with, but NixOS gets much easier the more nix you learn.
What do I mean about that? Imagine having to patch something, which can be the thing. On arch you’d have to replace a package, which could lead to issues and conflicts, whereas NixOS gives you the option to keep two or even more versions of the same library, because it does not rely on your traditional UNIX path.
But with this super power comes a catch. You have to learn a programming language and learn how the nix store operates, which is a pretty high learning curve. Also, NixOS suffers from a governance issue and going by the documentation is like shooting in the dark.
That being said, the best manual for NixOS is GitHub, searching for anything and filtering by the nix language. You’ll see a ton of varying systems, be they workstations or servers.
And no matter what all the warnings say, no, flakes aren’t EXPERIMENTAL or UNSTABLE, but rather CONTENTIOUS internally. Again: I love NixOS, but they gotta fix their governance issues.