I imagine VNC can't do this well because it streams pixels with no optimizations other than antiquated compression (it can't even match WebRTC screen sharing), and crappy color depth.
The idea is interesting for lightweight computers e.g. chromebooks and ultrabooks, but it would irk me a lot to have my browser and personal information running on some other machine that I don't control.
What I would be super-interested in though is a self-hosted version of Mighty, that I could install on a Linux box anywhere of my choosing. For example, the server runs on my powerful desktop at home, and my ultrabook in the bedroom can be a client.