I have a question regarding the migration from initd to systemd and a /usr volume.
When RHEL and Ubuntu moved from initd to systemd they required that /usr be part of the / or the system wouldn't boot.
This seems the way systemd was meant to function, see e.g.:

I know that a RHEL server with a separate /usr volume is not supported even if you get it to work.

I installed a SLES12 server yesterday and made a separate /usr (in an LVM volume group) and the server boots fine.

So, my question is, why :-)
What is the diff between RHEL/Ubuntu and SLES? Is it possible this could cause issues in the future?