Arch is especially nice because it has best in class documentation and vanilla packages. The installation is a bog standard chroot and use package manager like Gentoo had for ages or an old school Debian. Arch is nice because it’s as vanilla as a Linux distribution can be.
NixOS is the polar opposite of that: basically no documentation, weird behaviours everywhere, custom configuration, custom file system. Pretty much as far removed from Arch as something can be.
The installation on my laptop is on its 4th hardware refresh. Rsync or brtfs send to a new box, modify the partition UUIDs, rebuild the initial ram disk, and you're on you're merry way.
But after about 5 years I would start having problems on my Arch installs and realize that some config I edited years ago is now causing a problem. I have to see if I should revert to the default config or merge in changes and if I need a new workaround. Overall the system was just accumulating cruft that ended up biting me. I never knew what configs I had edited and what the default was, experimenting was annoying forms of backing up the config, making changes and then figuring out what I actually changed.
NixOS is completely different because all of my "customizations" live in a Git repo. I have change history and it is trivial to remove things that aren't needed anymore and know I did a clean job. The concept of a re-install even doesn't even make sense. Every config change or update is just as clean as a re-install would be other than "state" like a database or my documents.
There are definitely a lot of differences between NixOS and arch such as many packages requiring patches to function at all or as expected. But "never reinstall" was a significant part of my ramp from Arch to NixOS.
I've since migrated to a flake-based setup with machine-based variations (for my laptop and desktop), including easily swappable desktop environments. At a whim, I can switch between sway, hyprland, and gnome. This was mostly a result of me exploring/tweaking these without wanting to discard the configs; I always end up coming back to re-explore tiling WMs.
My experience through all of this has been great. I've even done a full re-install on both machines when the xz vulnerability was discovered and the process was effortless. That includes lanzaboote for SecureBoot, LUKS, and out-of-tree git-based flake builds for custom applications I build from source.
The one thing I found really helpful when starting with flakes was this repo that includes starter configs to help flatten that initial curve: https://github.com/Misterio77/nix-starter-configs/tree/main
NixOS is nothing like that, there's limited docs, the docs are sometimes just RTFM that links to another RTFM and oh yeah don't be afraid of breaking userspace because userspace is doing something wrong. It's fine for some of us who can of course get shit done.
Ultimately, I think some subset of Arch users who really wanted to be Gentoo users but weren't because "drama" and then NixOS scratches that itch.
