The difference is that upstream for upstart is open to BSD/Hurd/etc. ports, whereas upstream for systemd is pretty strongly opposed to them and it does not seem they would assist in the effort.

As long as you are willing to sign the copyright license agreement, which was one of the reasons systemd was started instead of improving upstart.

The upstream is GPL + CLA. From a Debian perspective that's something of a non-starter. You could fork it, but then you're in the same position as you would be with systemd.

Moreover systemd has defined a bunch of stable interfaces; one could choose to expose equivalent interfaces in the HURD rather than port. Upstart seems to be in the "implementation = interface" camp.

If I were to port systemd to non-linux I wouldn't fork it. I would re-implement its interfaces (which are fairly stable and well-documented).

systemd fairly strongly depends on linux cgroups for process monitoring and management, no? I don't believe there's a suitable analog in any of the BSDs.

BSD has jails and rctl for controlling users resources limits and other analogous tools for process monitoring. The issue is systemd was not designed with portability in mind. Taking full advantage of Linux's features is fine for some distros, but Debian's philosophy has always been grounded in stability and portability.