They don’t cover aspects of a desktop environment that are not related to using Mir. Missing functionality includes: integrating into the system for screen locking & suspend, policy kit integration, internationalization, etc.
The beta and edge channels track Mir releases:
$ sudo snap install --classic --beta egmde # Latest Mir release
$ sudo snap install --classic --edge egmde # Latest Mir development
To use egmde as your desktop, select it when logging in, to run it on your desktop run "egmde".