You are missing the point of Mg—it's not to replace Emacs as your development powerhouse, but to give you little binary that acts a lot like Emacs but runs in some tiny vm or embedded device somewhere.
You can also cross compile it (which you cannot do to Emacs).
It sounds exactly like all the various attempts people have made over the years to make a vim-alike textarea editor in javascript. They always forget block select mode, or don't implement bookmarks quite right, or something, which ends up in frustration if you're used to using your editor of choice as more than a Notepad.exe-alike.
You can also cross compile it (which you cannot do to Emacs).