Tbh, I don't really play DD that much but I have been following for a while.
But I thought I would chime in here because I think the discussion of "should all classes be able to beat all dungeons?" an interesting one.
If from the get go DD has established that all classes should beat all dungeons, then I can understand that a few of the players being heartbroken when this is not the case.
But from my understanding, DD has been about the difficulty of the challenge and having to "puzzle-solve" the dungeons. And maybe a reward for the hardcore players is to include dungeons that can only be beaten by one class. In that, it challenges the depth of knowledge of the hardcore player to know that a class may beat an "impossible" dungeon and to understand in-depth why only that class can beat that dungeon.
But I think in order to do that, you need to draw a massive outline around those dungeons and make the players aware that only one class can beat that dungeon. This type of "mode" could also be extended in numerous ways and perhaps even extending to a different set of challenges.
But I am going to put a massive disclaimer on the above opinion, in that I really don't play DD that much.
You guys could have already implemented that kind of system and I wouldn't know.