I don't understand usrmerge (and I don't like it) and I prefer to keep / and /usr on different partitions (with / in read only). / should contain all program required to boot a minimal and usable system. usrmerge prohibits this configuration and in embedded systems, a root partition in read only is mandatory (to restart a device even if /usr is totally broken after, for example, a power outage). That being said, in embedded systems, we wait for a rescue directory (like /rescue in NetBSD world) where all critical programs are statically linked.

In diskless configuration (my workstation is in nfsroot), usrmerge also leads to multiply nfs request. I have tried, a diskless workstation with usrmerge is slower that the same workstation without usrmerge.

In both cases, merging / and /usr is a very bad idea. For me, usrmerge and systemd are the exactly the same nonsense.