Mega-dungeons are great. I’m running one now and I basically have zero prep, I made the dungeon and just see where they go. They go in the dungeon and explore, get into some trouble and grab some loot. It’s honestly so nice to have them dungeon crawling. My last game I was juggling plot lines across 5 cities and making custom content constantly in prep. I was burning out between 5e and building content every week.

5e Undermountain is a very poor megadungeon.