Where's the front brake?
What happens when a sideways pressure is applied to either wheel?
How much weight will end up being added to the frame and wheels in order to keep them stiff? Removing tubes from the normal diamond frame design mean you need to make up for the support they provided elsewhere.
What I never understand is how people imagine the drive on the rear wheel. You'd need an impossible gearbox to spin the gear that interfaces with the rim, and even that gear would need to spin like 10 times for every revolution of the wheel.
To me, that kills the fantasy so thoroughly that it isn't even fun to look at a picture of the device and imagine it in perfect world scenarios.