POA/POI shift can range from negligible to severe with no real rhyme or reason (just not something the optic is designed for) when mounting forward of a daylight optic.
Having not watched the video/DVD, however, I'm guessing he's probably talking about mounting behind a daylight optic.
The biggest issues that you will run into is the obvious fact that unless you have another monocular head mounted - you need to point your weapon at anything you want to look at, and also the fact that under most circumstances, a daylight optic in front of the NOD will almost invariable result in some obstruction of the FOV, which is already fairly limited with most conventional NV monoculars, meaning not only do you have to point your weapon at what you're looking at - you can't see as much.
What he meant by "proven in court," I cannot speculate--but beyond a limited number of circumstances where a rear mounted monocular might be useful, you're somewhat needlessly giving up a lot of situational awareness and safety using an exclusively weapon-mounted monocular.
~Augee