Resource management in KDE

cgroups v2 is the intended successor, hybrid hierarchy is only a transition.
New cgroup-related features are only happening on v2, both in the kernel and in systemd.
The "processes only in leaf nodes" rule, while a constraint, actually makes things easier to manage and reason with.

This page https://systemd.io/CGROUP_DELEGATION/ has a lot of details on cgroups v1 vs v2 from a systemd point of view.
Highlight: "To say this clearly, legacy and hybrid modes have no future. If you develop software today and don’t focus on the unified mode, then you are writing software for yesterday, not tomorrow."

Another limitation is that managing user resources through systemd will never be possible with cgroups v1 / hybrid hierarchy, as this would require delegation to the user systemd instance, and "Delegation to less privileges processes is not safe in cgroup v1 (as a limitation of the kernel), hence systemd won’t facilitate access to it."

Also, docker (git master) finally supports cgroups v2, which should be in the next release (https://github.com/moby/moby/milestone/76). AFAIK, most other container runtimes have had support for a while.