I have done one semi-mega-dungeon in 5e. I agree with most of what's been said about what made the campaign fun - varied encounters were important for our group. I resolved to give up complete internal consistency in favor of variability. That flies in the face of some ecologically sound dungeon principles, but I think it depends on your group. Even if an entire level should be dominated by the hobgoblin tribe, after three or four hobgoblin encounters it can turn into a slog. The other thing I did was have lots of mini goals and bigger goals for the PCs that had rewards besides treasure and XP. Probably the non-traditional rewards I used the most were favors from powerful groups and access to new areas. YMMV and all that.I am considering building out my next campaign as a mega-dungeon exploration (with a side of boom town political drama, but that's not important right now).
As I do so, I want to hear about your experiences playing or running dungeon focused campaigns. it doesn't have to be a mega-dungeon campaign, but it should be dominated by dungeons. What elements of play did you find helped make the game fun? Which ones made it tedious? If you were a DM, how much did you prep ahead of time? If you were a player, did you notice how much the DM prepped ahead of time? Was it/were they published dungeon(s)? Was it all home brew? A mix?
I prepped a lot, but mostly creating terrain, props, and painting minis - all completely unnecessary, but fun for my group.
We went from levels 1st -20th and it was a heavily modified version of the Dungeon of the Mad Mage. It worked so well for my group because they love combat. I would not attempt with a group that does not have combat as their favorite part of the game.