I haven't used NixOS outside of VirtualBox, but plan to use it for my next Linux setup.

I believe the reason for build-vm is that systemd is central part of NixOS and you can't test it by starting it inside of a container (I don't think systemd even allows it).

The nixos-rebuild command actually creates a path with a new configuration, so you could similarly use nspawn and go there. When you use "nixos-rebuild switch" it updates "/run/current-system" symlink to that new location.