Still not sure if the filesystem layering was really a necessary part of containers. Having built thousands of docker containers and managing deployment pipelines I noticed how hard it is to actually utilize the layers effectively. Almost any change can easily blow the cache of a docker build. Usually you should be updating the OS frequently anyway for security reasons and caching it can lead to laziness on updates.

Systemd nspawn uses bind mounts and isolated filesystem access (like a chroot) which is nice for performance. It is a slight paradigm shift on what a container means, so far so good though..

One thing I've been really liking so far too is you can Boot your containers for multi process... For my use cases this is way superior to single process. For example, haproxy does hot reloading by managing processes. Additional processes managed by systemd from inside a container are also really handy.