Doesn't the short flange distance make it harder to produce wide and/or fast lenses?
They were able to do it on a smaller sensor, EOS-M with 11-22 and 22mm F2. It'll be easier for an FF version.
As far as I understand, it's the other way around - the farther the pixels from the optical axis & the shorter the flange distance, the obtuser the angle the light hits the pixel, causing vignetting. As far as I gather, this is already an issue on current FF cameras with fast and/or wide lenses, e.g. the 24mm f/1.4 L II has >3 stops of vignetting when wide open (translation: corner pixels record ~1/10th the amount of light center pixels record).