> The moment you need to start comprehending the difference between Wants vs. Requires and not hooking into running when multi-user.target is reached (the most common situation) you are in for a world of trial-by-error testing.

And the worst is that the tooling to test is very limited ( non-existent). I had a problem with conflicting/circular ndependencies ( my service had to be launched before networking but after dbus, which is just impossible because dbus needs networking itself), which worked 9 times out of 10. It was nearly impossible to debug, i had to use the graph tool that shows you visually how much time each service took to start, and completely by chance i noticed that dbus started after networking, which led me to look into it.