I've heard that too. I have zero experience with these optics so have no firsthand info to relate. Some guys seem to like it and others not so much. From the responses on ARF it seems the optics have some other issues such as excessive lens flaring with pics showing it. There is another thread (link below) with more info. The original poster felt the NVG-50/40 was bad enough that he deleted all of his content but there is other info in the thread.
NVG-50 2nd thread
IMO, I think it's a better investment to spend a little more and buy a more popular/common housing that uses actual PVS-14 optics. I believe they raised the price and resale is something to consider because you'll take a bigger hit on that setup (compared to something like an RNVG) if you decide to sell it down the road.
Were you looking at that housing because of gain control or just the wider view?