I never knew either dungeon to be challenging, honestly. AV was heinously tedious - and the Vault was actually fairly enjoyable. But difficult is not how I would describe them.

I would actually favor engagement over 'difficulty' which I felt some dungeons achieved in Stormblood, Ala Mhigo among them.

The thing is, challenge, difficulty, and engagement is all subjective on the viewpoint of the player - so I don't think using the terms we're are applying currently is at all constructive. Maybe we need to change our approach and disseminate what we can agree we want out of dungeons.