Well I spent most of last night Googling.
I think this might the root of the problem:
http://overclocking.guide/asus-rog-crosshair-vi-hero-extreme-overclocking-guide/#Known_issues
I think what might happen is that every once in a while, the ratio gets applied first and for a split second, the memory tries to run at a high frequency with low timings and hangs.
Also, who knows at what time during the POST process the higher memory voltage gets applied. It might be that the memory initially tries to run at 1.2V for a short while, until the 1.35V that most high performance kits require can be applied. This would explain why I have POST problems but the memory is stable once the system has booted. In that case, memory that is sufficiently stable at 1.2V to at least POST and let the BIOS take over would probably work normally.