NixOS doesn't follow the UNIX philosophy in many ways, especially when it comes to systemd. If you want NixOS you will have to use systemd and embrace it. Otherwise you can go the Guix route and try to make the most out of the much inferior ecosystem.

NixOS does systemd right:

- systemd gives the use a unified API for working with services. The journal etc. - Nix gives the programmer a nice way to define systemd units.

If you hate systemd due to normal Linux..NixOS might change your mind.

Officially yes. Unofficially there're have been attempts to support other init systems, e.g. https://sr.ht/~guido/nixos-init-freedom/ (s6), https://github.com/svanderburg/nix-processmgmt (abstract; allows a few inits).