systemd tries to unify a Wild West situation where everyone, their crazy uncle, and their shotgun-dual-wielding Grandma has a different set of boot-time scripts. Instead of custom 200-line shell scripts now you have a standard simple syntax that takes 5 minutes to learn.
Downside is now certain complicated stuff that was 1 line need multiple files worth of workarounds to work. Additionally, any custom scripts need to be rewritten as a systemd service (assuming you don't use the compat mode).
People are angry that it's not the same as before and they need to rewrite any custom tweaks they have. It's like learning to drive manual for years, wonder why the heck there is a need for auto, then realizing nobody is producing manual cars.