So look at how Lennart Poettering crippled udev and how he writing programs/daemons. He does not care about unix standards and other unix systems (bsd, ...).

Really now when udev is part of systemd, /usr and / must be on same partition (or /usr must be mounted before starting udev). But in systemd mount process is done after starting udev which creating files in /dev (also device for /usr).

And now all linux distributions (except small epsilon) depends on udev...