It's a stunning piece, especially live with the acoustics of a schimmel in a concerthall. I love his compositions in general, especially Midwayer, but Hanging D was the only one to give me goosebumps. It's a very atypical but simple composition though. Everyone with at least two fingers on each hand can learn to play this piece within a few days and master it in a few weeks. So, is it underrated? I think that's very debatable. Beving uses the piece to end his concerts, just as an impressive ending to a beautifully relaxing concert. The piece guides you back to reality after dreaming off. And I think everyone who will experience this piece for themselves will appreciate it for its aesthetic; it's beautiful and powerful. On the other hand, this isn't translated very well through a recording. Listening to this piece on a recording doesn't sound nearly as impressive. I had some people have a listen to the piece who totally didn't understand it, but I bet they would once they hear it live. I also noticed that when I perform the piece on a public piano, people are often curious to ask what I played. Despite his growing popularity Beving is still an underrated composer in general, so relatively seen I wouldn't say Hanging D is specifically underrated. Personally, I think Midwayer is by far his most underrated piece.
