Systemd support seems decent enough, so this doesn't seem like an important feature anymore.
It's surely heavier than an S6 based solution, but a systemd-based solution seems more likely to work correctly. This is certainly not an inherit property of systemd. Rather, this has to do with the lack of an interface in NixOS for doing things without systemd, which results in tight coupling with it.

A safer path seems to be to modularize NixOS, so you can leave out parts like filesystem support and networking. Another improvement is to compile a systemd with fewer options enabled.

It's easy though

Of course, if anyone wants to contribute a PR that adds support for an alternative init system, that would be very welcome, and I believe that the way service configuration is set up should support that - just have a look at the nixos-init.nix module for an example. It's tiny, little more than a mkIf and image.command = [ "${config.nixos.build.toplevel}/init" ];. A new module could do the same, but differently.

from arion.