This is not only about the willingness to tolerate ifdefs. I said earlier that porting systemd to kFreeBSD would be a daunting task, and that was probably an understatement. Poettering had this to say about the matter: https://lwn.net/Articles/524920/
There are real technical issues going on here, to the point that "porting" systemd to kFreeBSD would mean to essentially rewrite it. At least that is the impression that Poettering's comment left me with, and he should know given that he wrote the thing.
Of course, things would be different if FreeBSD were to adopt the essential Linux APIs that systemd needs. But I don't think they will as most of the functionality is already there, just in different form. Linux has devtmpfs/udev, FreeBSD has devfs/devd. Linux has epoll/inotify/fanotify, FreeBSD has kqueue/kevent.
Nevertheless, it *might* happen, just like we *might* one day genetically engineer pigs to grow wings and fly. But it's coffee cup reading more than anything else.