fleet definitely does not provide a nice integration with docker, and we're definitely trying to decide how we can provide a better integration story. And yes, systemd is complex, but with that complexity comes great power.
For example, etcd is a powerful primitive, and then more complex/sophisticated systems can be built on top of it.
I wonder if an 80/20 solution that is simpler than fleet/systemd for pushing work into a CoreOS cluster would be a win, and then more complex systems (e.g. Kubernetes-esque orchestraction) could live on top of that.