I suspect the problem is that hydrogen would interact with and erode the metal in the heat exchanger.

* Power density is limited by the heat transfer rate of your heat exchanger. This tech isn't very useful if T/W is less than about 5.

* Launching to orbit requires a straight line acceleration run of 0.5 * (planetary diameter) * (planetary surface gravity) / (acceleration); on Earth, about 3200 km for 1G. If we launch at 3.2G, we still need a 1,000 km focal array. Required (diffraction-limited) focal array size is equal to (desired range) * (wavelength) / (target size). If the target heat exchanger is 1m across and we're using 1mm microwaves, that requires a 1 km focal array.