You’re complaining about something different. If there’s a bug in systemd itself that makes your system unbootable then not much you can do except not upgrade. What the parent is talking about is the fact that services are organized into base service files and overrides. Base files live in /usr and are to be modified only by the package that owns them. Override files or full-replacements live in /etc which is for the local administrator. Service updates are now always a safe operation.
lol. I had to trash an Arch Linux box because it failed to upgrade systemd from 208 to 211. Everytime I upgraded the system hung trying to mount filesystems so I had to roll back.