I'm running a megadungeon at the moment. The party are in Hell, though they haven't yet experienced much hellishness. Yet. Muahahahahahaaaaa.... and all that.
The reason for my lukewarm acceptance of this model of mapping is purely to do with manageability. A map like this is going to require pages and pages of room and encounter descriptions, even if they're pared down to the bone.
That means a lot of page-turning, and inevitably there will be times when numbered encounters, which may be adjacent on the map, will be pages apart in the written descriptions.
That adds a level of confusion I don't need.
Additionally, the sprawling nature of the map means that the party could go just about anywhere, any time, which makes it that much harder for me to plan ahead, or to take into account relationships between dungeon denizens on the fly, or to foreshadow upcoming event possibilities. I don't mind having to do a little prep work as DM, but I prefer to keep it as minimal as possible.
I can fit a whole map on a single A4 page at a scale large enough to read easily and to make legible notes on, and I can fit the written descriptions for the whole map on a single facing page.
The modules can be easily interlinked just by noting where the exits are, and how each exit connects with any of the other maps. They could all be part of the same level, or they could be scattered across dungeon levels.
Best of all, wherever the party goes, the map and description are right there together — which makes for excellent manageability. The end result is transparent to the players; as far as they're concerned, all of the maps might as well be one huge many-folded confusing mega-map.
And if I decide, on the spur of the moment, to introduce an area in which all the tunnels are made of rotting meat, swarming with black beetles, worms and maggots, it's a piece of cake to introduce a new map into the ecosystem.