I visited Pohyon temple and the Mt. Myohyang area during my North Korean tour in late 2017. As with all things while in the country, I had no control over what we were doing but as a main part of our itinerary on day 3 we drove to this area primarily to visit the International Friendship Exhibition Hall, tucked away in the mountainous area across the river from the temple. After a while of viewing the displays of gifts in a few rooms (some of the gifts were impressive but most seemed to be from Chinese companies (greasing the wheels of business?) or unheard of fringe communist groups in random countries whose membership probably doesn't reach triple digits...) we head over to the nearby Pohyon temple.
Unfortunately by that point it had started to rain and we were rushed around the grounds rather quickly with only the briefest of commentary. Not that I would know what to believe if they told me more. The area is scenic for sure and the temple grounds extensive and well kept/restored, after being bombed by the Americans during the Korean War, with many other small halls, gates and shrines scattered about. In any other situation I would have spent more time poking about or backtracking a bit once the rain eased up somewhat.
The guide claimed that there is freedom of religion in the country though I suspect their definition is much more limited compared to home and indeed there was nobody else there at the time other than some very elderly monks. Whether this is due to its relative isolation or other reasons I don't know. As far as OUV, I can't say much. There are many scenic and sacred mountains already inscribed and we can't do them all or it will be a very long list indeed. The cultural aspect is no doubt more important as it is one of the oldest temples remaining in North Korea and although it is considered a national treasure it is not tied to the current dynasty so who knows what they will do with it.