Storage needs to be detected on boot before the init system can do anything with it. That's going to effect when you can bring up other daemons, including network ones. Which in turn may require bringing up network connectivity so you can mount network filesystems that you may be booting from in the first place.
All of which needs to be done in accordance with the idea that we might not be booting, but instead coming out of hibernation or sleep mode, and we need to check that the world wasn't exploded around us while we were doing that (i.e. the network is still there, is still the same network, and the storage is still reachable or hasn't been hot plugged).
If you don't think cgroups are the responsibility of the init system though I don't even know what to say to that. That the process which launches all other processes shouldn't be able to request they be isolated, and monitor them to check they're still running?
Frankly, I don't know why people think these things aren't the domain of the init system. They are all absolutely fundamental activities of a modern computer system, which need to functional at the various levels of boot time when nothing else may be running.