I tried using systemd-nspawn as an alternative to Docker (not because I don't like Docker, but because trying alternatives is cool) a few years ago and I failed miserably. The docs were hard to grasp and at the time the concepts of namespaces and cgroups were a bit obscure. I guess there are more docs and blog articles making use of systemd-nspawn nowadays, I'll have to look into it.

This showed me how great and easy and well designed Docker was as an abstraction layer though. I know purists don't like it, but it made reliable deployments standard and easier than the alternative of not using Docker.