That is like saying Apache project is subject to not enough review and unrelated subproject bitrot.
https://projects.apache.org/projects.html
or GNU
https://directory.fsf.org/wiki/GNU
Or FreeBSD or OpenBSD....
Having a single large overall project that manages a diverse number of individual software products is a common pattern and certainly something that isn't remotely unique to Systemd. There is no reason to denigrate systemd for that.