I've got mixed feelings as to how hard it is. I suppose with the M8/M9, to squeeze a quality level out that is appropriate for the Leica logo, it was extremely hard and maybe due to the short back focal distance of all the Leica RF lenses.
The Epson and 4/3rds cameras were produced with sufficient imaging qualities - maybe Leica wouldn't think it was good enough for them.
I'd love to have a digital back for my M's, that was affordable. And had the same specs as the M9, or better.
Vick