Author here. Your TLDR is spot on. Yes, my intent was to be on desktop use since most things I read dont consider that specifically. I did talk about how I would keep this running on some simple home servers since I think that's where Nix shines. But some of my servers are raspberry pis, which I mentioned I am worried to run Nix on due to resource limitations. I should probably just try it.
I'm running NixOS on a raspberry pi and I deploy to it with deploy-rs¹. This works pretty well for me. My dev machine is an Apple Silicon laptop with nix-darwin installed and I use its nix.linux-builder module to run an aarch64-linux VM as a remote builder to build the rpi's system. All this means the rpi never has to do any building itself, and doesn't even need the nixpkgs source installed either.
If you want to do this yourself, I recommend using https://github.com/nix-community/raspberry-pi-nix so the system is configured much more closely to how the stock raspberry pi image works. The benefit of this is better reliability of stuff like bluetooth.
I wish remote build/deploy for Raspberry Pi was in a better state - it seems like a perfect fit for NixOS.
I've got x86 servers running NixOS that are deployed using Colmena, but it seems to fall apart when I add cross compilation into the mix.