as someone who does not really understand it, it does look non trivial:
"""
Which Linux-specific functionality does systemd use and what's the advantage of this over a solution that is portable to other operating systems?
There's quite a bit of Linux-specific functionality we rely on. The first that comes to mind is probably cgroups (short for "Control Groups") which is a Linux kernel feature that may be used to create hierarchical groups of processes, label them, and optionally apply resource limits or other rules to them. But there's a lot more. Systemd tightly integrates with the Linux-specific udev system for hot-plug events. And then, the disk read-ahead solution we include in systemd uses the Linux-specific fanotify() call. Or we support spawning processes in their own file system namespaces or with limited capability sets, both of which are Linux specific features. Or we support the Linux-specific automounter and other mount APIs such as polling mount changes via /proc/self/mountinfo which do not exist on other OSes. Internally we use a lot of newer API calls, such as timerfd() or signalfd() which are not available outside of Linux. These latter calls could be emulated on other OSes, but using them greatly simplifies our own code. And many of the other Linux features we use have no suitable counterparts on other Unixes at all.
"""
Which Linux-specific functionality does systemd use and what's the advantage of this over a solution that is portable to other operating systems?
There's quite a bit of Linux-specific functionality we rely on. The first that comes to mind is probably cgroups (short for "Control Groups") which is a Linux kernel feature that may be used to create hierarchical groups of processes, label them, and optionally apply resource limits or other rules to them. But there's a lot more. Systemd tightly integrates with the Linux-specific udev system for hot-plug events. And then, the disk read-ahead solution we include in systemd uses the Linux-specific fanotify() call. Or we support spawning processes in their own file system namespaces or with limited capability sets, both of which are Linux specific features. Or we support the Linux-specific automounter and other mount APIs such as polling mount changes via /proc/self/mountinfo which do not exist on other OSes. Internally we use a lot of newer API calls, such as timerfd() or signalfd() which are not available outside of Linux. These latter calls could be emulated on other OSes, but using them greatly simplifies our own code. And many of the other Linux features we use have no suitable counterparts on other Unixes at all.
"""
from http://fosdem.org/2011/interview/lennart-poettering