I'd compare runC to LXC as opposed to rkt. It just takes a path and configuration and starts a container, without managing images or networking topologies. rkt is on the same level as cri-o (or containerd) since it also implements image handling as well as other related things.

Also, rkt (by default at least) uses systemd-nspawn to create containers. In runC we have our own implementation called libcontainer.

runc is lower level. Docker uses runc under the hood to run each individual container, and rkt is somewhere in the middle as far as features go

http://jvns.ca/blog/2016/10/02/i-just-want-to-run-a-containe...

rkt does not implement the OCI runtime spec, which is what runc implements. It supports the OCI image spec.