A friend of mine who was approaching AR from a more theoretical point rather than a product point shared some of the physics of how many nits of brightness the display needs to produce to occlude the actual background and it is a lot. ML worked around some of that problem by shading the background think sunglasses with a 50% light reduction, which helps but then you're trying to match shading. Basically it is a really hard problem and had they not been so "out there" in their original claims I really don't think they would be so challenged in their marketing now.