Some of you can know me, i'm playing on BGTSCC since a few months, making a ton of chars sadly (

Other than this UD maps are really good, Sshamath is creepily wonderful, and i thik is a shame to see so few players there around. From my LITTLE experience, playing a drow or a duergar even from the 1st level is hard but not that difficult after all... only slow.
But i believe the survivability of the UD is pretty decent, with enough CR 1 areas around (one even below the Inn!!!!!). The only serious issues to survival i've felt were with wizards and sorcerer chars, others are really simple to make grow in experience levels.
As a DM said to me some time agoo, i'd want too to see more dedicated players for the Underdark, because FOR ME is not an hardcore place, even wandering SOLO...
Sorry for this little digression, but i really needed to write this about a place really good made and with so tremendous potential.