in systemd runlevels are basically just targets (it still sets rc?.d symlinks in /etc akaik) which have services they want and are wanted by, it’s the basis for dependency handling plus you get cool security features like syscall filtering, capability limits, user switching, etc

You can end up with a lot of boiler plate code and with duplication you run the risk that one unit tweaks the boiler plate in a way that behaves differently. This isn’t insurmountable and a lot of rc scripts source a library of common functions shared between units. However from the point of view of the executor each unit is it’s own whole ball of shell script code.