Updating to more current systemd releases is, generally, safe. However, we're going through a ton of transitions right now, including the cgroups refactor, the move to systemd-bus (to prep for the kdbus move), and network management. While none of these explicitly break backward compatibility, they have introduced temporary stability issues. I'd worry about tracking upstream too closely for a server OS. Also, RHEL will have to have some answer for this, too.