Which of course still leaves OS X, *bsd, Solaris AFAIK. But it should cut down on the internal Linux incompatibilities a bit. I'm not a fan of systemd, but having one broken standard is much better than having four broken standards :-)
AFAIUI OS X has launchd, and Solaris has OMF[1]?
... so basically the BSDs are the ones left behind?
[1] I think that is the acronym for Solaris' XML-based standardized system service files? (I don't care that they're XML. It care that they're standardized.)
Here's a "portable" script for van Smoorenburg rc that ports to 4 Linux families, complete with the sort of case statements that I described in https://news.ycombinator.com/item?id=10357589 :
* http://conman.googlecode.com/svn/trunk/etc/conman.init.in
Here's a Mewburn rc script:
* http://cvsweb.netbsd.org/bsdweb.cgi/~checkout~/src/etc/rc.d/...