[jakobegger]

> ... serious engineering challenges ...

I'd say there are serious physical challenges with this problem. Wireless power transfer over distances of more than an inch without big parabolic antennas is about as realistic as hoverboards.

[georgespencer]

uBeam is the canonical example of this. Their own VP Engineering left and publicly stated that the technology simply doesn't work. ELI5 is that you need an impractically large phased array to transmit, and the attenuation is precipitous even at short distances. The commentary I've read (by no means exhaustive) treats it as if it's a "laws of physics" problem, but I would assume the company had some reasonably compelling answers to this challenge given that they raised $40m USD from some bright investors.