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.
Glad to hear you're thinking about this. Understanding the complexity provides power, but the tradeoff in this instance doesn't feel quite right.
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.
I took one look at Kubernetes, and decided that for our purposes writing our own orchestration scripts from scratch would be simpler... So many of these systems are hopelessly overcomplicated.
IMHO, the weakest part of CoreOS is fleet (https://github.com/coreos/fleet). Compared to the other components in the stack, it just feels very inelegant. The systemd configuration syntax is complex and ugly. I wonder if there will be work invested to upgrade fleet to something that is as elegant as e.g. etcd/Docker/CoreOS itself.