Yeah, init scripts are simple, but things get tricky outside of the happy path. Think of service upgrades/restarts. Once you care about dependencies, then you are back to a graph like systemd's.
systemd is awfully documented (volume is not quality), things are still changing and haven't even caught up in functionality with any init hack.
the only saving grace of systemd is that it almost makes it easier to use namespace and other sandbox (but only because those are newer and init systems also didn't have their solutions up yet)
systemd only purpose is to mimic windows administration in linux. sorry of anyone who fails to see this
When are the init systems going to implement the basic security measures systemd has?