vs flatpak vs AppImage vs Snappy

Flatpak definately uses all those features, as well as shipping all the dependencies (up to and including libc). Thats not a full "OS" though, as it doesn't have the host support stuff like init, hw support, kernel, logging, etc. Just the libraries.

Snappy is somewhat similar, but it doesn't use namespaces, but instead multiple prefixes that get set up with env-var munging + apparmour to hide away things the app shouldn't be able to access.