I never saw Underdark being a low level adventure. In any computer game or P&P module.
"Out of the Abyss" - is an offitial WoTC module that starts in Underdark at lvl 1.
You are stuck in a mindset of leveled zones. Which is not how DnD areas work. They provide environments and possible problems. Which parts of it player's encounter is DM's decision.
Is not "leveled zones". Is
ADVENTURE LEVEL. Some Dark Sun modules recommend starting at lv 3 or even 5. but you can start at lv 1 as longs you stay most of that time in the city. Most Ravenloft(Note : Ravenloft is not only Strahd, Strahd is just one of the many dark lords) recommend starting lv 8. If you wanna put your players AGAINST Meredoth, you need to let then be at least lv 15 to have a chance. If they only wanna to escape the realm of dread somehow, then they can probably be around lv 8~10 like the ship of horrors recommends to start. Same location, but two different adventures IE - Escaping a realm of dread VS killing a lv 20 Dark Lord in his own domain. Same location, but two different "goals" with two different difficulties.
(...)first second you step over the border of Underdark, horde of Undead, Vampires, Drow, Duegar, Mind Flayers, and other abominations ... will attack you? :-/ Come on man, surely even Underdark is big enough to have at least few caves that are more or less, well at least not as deadly as the rest, if not exactly "safe".
Still Underdark in BG3 is one of most chalenging parts of Act1 so far ... maybe except Githyanki patrol. :-/
In some settings you don't even need to step into the Underdark. Mindflayers, Drows and other things will attack and raid the surface...
