Yes, but it shouldn't be damn near impossible to get to either it takes away the fun of the game especially when you've got a matriarch with 6 times your hit points and can take out a 3rd level in one hit.
>>Fastfowarding >>
I'm not saying that the game has to be easy but it doesn't have to be overkill either because both of these ways make a game for which can not be properly enjoyed.
So you talk about the hardcore entrance that was shown to us also in the developer stream back then.

You dared to fight THAT THING with a lvl.3 group? I knew I'm a pussy. ^_^
I would never dare to face that thing with anything but lvl.4.
And also my tactic would be to set that thing on fire and just throw smokepowder barrels.
Maybe Arcane Missles for a Mage but everyone else utterly needs to throw these C4.
But I am also an Arachnophobe so I want it dead asap anyways.
You know that below-the-village-entrance is the hardest of them all, right?
I believe you also need featherfall otherwise your group will arrive in the underdark as corpses I believe. As in it is a 200 meters fall or something.
This is the hardest entrance.
I think the easiest entrance is the one in Ethels cave.
If you kill Ethel, which is fairly easy I would say and wander through the illusionary wall in her cavesystem, you arrive in the underdark pretty easy.
But beware if you still have the unique debuff just for laying eyes on Ethel.
This landshark thing is using acid attacks, which are amplified by the debuff to oneshots.
But in general I believe this is the quickest entrance to the Underdark.