Large dungeons are not impossible in Torchbearer. But, you need a disciplined party that can act with precision. You need strong leadership and creative thinkers. You need the ability to plan ahead, to understand when to take checks, when to pour resources into a chance at success and when to camp and regroup. Luck doesn’t hurt either.
While death is decidedly on the table for characters in Torchbearer, it generally doesn’t come quickly–this is one of the major points of departure from early D&D. In Torchbearer, you can get beaten down to the point where your character will probably wish she was dead because the laundry list of conditions she’s accumulated feel pretty debilitating. To quote one of our play testers in a session after running into monsters tending a massive cauldron on a fire: “I just get in the cauldron.”
This tends to come about because parties overestimate their capabilities, underestimate monsters, or push on too hard for too long without finding a place to rest. You can be doing fine, and then before you know it, a few poor decisions will snowball on you and you’ll find yourself in a world of pain. Combine this with the fact that novice players tend not to think about accumulating checks until they’re already in trouble condition-wise, and, well, you see where this is going…
This happens less frequently in a small dungeon because you’re likely to finish it before you have all your conditions marked, and thus the game tells you it’s time to head back to town. In large dungeons, there’s almost always plenty of more stuff to deal with and the players have to be self-directed about determining when to camp.
Long story short, go ahead; take them to Undermountain. I want to hear about it! I just suggest letting them cut their teeth on smaller dungeons first.