I have found them to be highly uncomfortable. I’ve taken several night trains in and around Central Europe and the rooms are claustrophobic, and god help you if you are in a four person. There’s barely any room for two people to stand. I’ve had arguments about temperature control where a lower bunk wanted air con off as it blew on their face, leaving it about 40c on the top bunk in summer overnight.

In Finland I’ve taken one of the more “luxurious” cabins with two bunks and a tiny shower. It’s marginally better but the walls are made or paper and the train stops during the night and you are awoken by passengers coming and going and worse the beeping of doors closing. This is with silicon earplugs.