I've been using Nix and NixOS for a while and I've seen four core advantages:
1. Nix store: what the article covers: multiple versions of the same package, "virtualenv for everything"... etc
2. Reproducibility: because package definitions are self-contained, I can pin and reliably reproduce everything for my packages. And I do mean everything.
3. Flexibility: Nix works for system packages on NixOS as well as user packages on any Linux or macOS. I've been sharing like 70% of my system config between Linux and macOS, and I don't know of any other tool that can do that.
4. Programmability: package definitions and config are extensible and composable. Yes, learning an ad hoc language to do this is a pain, but once I learned it I got a higher-level and better-abstracted way to define and combine packages than any other tool I've used. It's like going from C to OCaml. At this point I've started hating Dockerfiles because they can't easily be composed and don't provide any real facilities for abstraction.
Ultimately, Nix took system management from feeling like an operational activity—maintaining and changing a bunch of mutable state to run the system—to declarative programming. Learning curve aside, making system management a reproducible programming activity with no system state to worry about has been a massive improvement.
"... with no system state to worry about"
I don't disagree with you, but one of my skepticisms about this is that Docker and Dockerfiles made the same promise like eight years ago, and at least as a small time operator (eg, homelab), it never really felt like the system was as stateless as it was telling me it was. There were always logs, indexes, caches, database content, and all the rest of it, and all of this had to be carefully volumed-out with those data containers being protected and versioned and carried forward independent of the throwaway Dockerfile-defined "application" containers.
So far my experience of NixOS is that it doesn't particularly offer a new solution here, but at least it doesn't pretend anything. The configuration.nix "defines" the whole system, but obviously doesn't touch anything in /home or /var, so that's on you to back up, migrate, whatever.
https://github.com/nix-community/home-manager
> or /var, so that's on you to back up, migrate, whatever.