It probably work best on FF and larger sensors where an extra cost of a modular system do not matter as much. But as it's the sensor modules that will be most expensive part, it's not much money to be saved for the user.
One problem is that it will not be easy to design a back that can be used for everything from Q to 645.
You need completely different ergonomics for these different cameras. For 645 support the back would probably have to be the size of Hasselblad x1D, which will be very large for use of Q and K-mount.
Compact Camera Meter