Let me elaborate. Good Programmers write Portable Code. This means checking for supported features and using them if they exist, falling back to less desirable methods otherwise. So either the systemd guys are too lazy to do this (and therefore I probably don't want to run their code even if my OS is supported) or there is something linux has exclusively, that is so damn awesome that systemd would suck if it didn't have it. To my knowledge, group scheduling is not that awesome. In fact, as far as I know, freebsd's kernel and userland apis are superior to linux's in many ways, and if this is false, I would like very much to know why.
I was tempted to call gnome and systemd lazy, but thought I should learn more first.
Full disclosure: I don't run gnome (apart from gtk) and probably never will, but only as a matter of personal preference, not philosophy.