I've been reading through how many NixOS modules use systemd units and there's a lot of scripts being executed, the final line execs the service (if there is one, NixOS uses systemd for maintenance tasks, refreshing certificates and many more things). While NixOS doesn't speak for the broader community what I'm trying to say is that it can execute anything, if that's a script or a daemon doesn't matter as long as it works for you.

Thanks for the sysV explanation, it sounds worse to me.