Systemd has taken an all or nothing approach for its components, and it has enveloped several significant components such as udev/upower/udisks. What this means in practice is you either have to take all of systemd (i.e. replace your init system, syslog, etc.) to use any of the components it has absorbed or you need to fork and maintain what you need yourself.
Here's a personal example: I use Gentoo an MATE as a desktop which in turn uses upower for suspend & hibernate. The latest version of MATE requires the latest upower (now dependent on systemd) to support those functions. So now if I upgrade MATE, I have to either replace my init system (OpenRC) with systemd or not have those upower features on my laptop.
Forcing their users(or distros) hand like that is not playing well with other software and I applaud Busybox for standing up.