I'm confused.
Etherealness says "up to 8 hours" which implies it can end at any time.
It also says: "You ignore all objects and effects that aren't on the ethereal plane, allowing you to move through objects you perceive on the plane where you originated from." So if a DM suddenly had some magic doors or walls, go through the floor or ceiling. If those are blocked throughout the entire dungeon, I don't know what to say. I'd call BS.
As far as gathering information, it's up to eight hours of information! You could literally watch for hours. I would say that would give you a lot more than a layout.
I could get behind someone blocking a room or a creature seeing you. But even then, that's pretty rare. But what do I know? I tend to keep my dungeons oriented towards the logical side. I hate it when there is a ghost, something that can literally move through walls, and it never leaves to see the goblins running around, and besides the goblins is a bullette digging around, and by the front door there just happens to be a gelatinous cube.

(And yes, those looking to logically make that happen can. But the dungeon better have a 20-page backstory in my book to back it up.