As a FreeBSD user and a Debian user, I was excited to try Debian/kFreeBSD around 2012ish? but it was kind of painful -- a lot of important FreeBSD userland tools weren't available in packages, so you had to do a lot of messing about to get things you might need. It might be interesting to run a Debian/kFreeBSD jail inside a regular FreeBSD install, but at that point, you may as well just bite the bullet and do things in regular FreeBSD.
It should be noted that a) FreeBSD is doing work to keep software working without SystemD (an obvious point, I know, but it might not be obvious that this means that work also benefits Debian) - so long as we don't see any crazy dependency on SystemD from Apt or the installer itself (hopefully unlikely) - there shouldn't be any issues. b) Debian/GNU Linux still supports more than one init-system -- at least for now.
While the long fingers of SystemD reaches for more and more of Linux, as long as developers are supporting other systems at all, there should be a way forward for distributions that doesn't want to incorporate SystemD. So this includes stuff like native Windows support, building on OS X, and packaging for the various BSDs -- all that work that goes into the big packages should continue to benefit Linux users that want to avoid SystemD for one reason or another.
> I was excited to try Debian/kFreeBSD around 2012ish? but it was kind of painful -- a lot of important FreeBSD userland tools weren't available in packages
As far as I know, this should've improved quite a bit. The biggest bar for entry (last I checked) was that the nightly installer builds might not work out of the box (one possible workaround would be to install Wheezy/7.0 and then dist-upgrade).
To be clear, I've yet to play with Debian/kFreeBSD, so this mostly from impressions from the list (and I'm not quite caught up with the latest). AFAIK the installer should now support installing on zfs rootfs, for example.