I get the feeling BioWare was given feedback to make the second expansion to Neverwinter Nights give a more Baldur's Gate 2 -like experience. A talking sword, ability to travel between places via a pocket plane, increased party size, and NPC romances (among other things) certainly hint to this.
HotU takes place during the silence of Lolth and some drow matriarch decides it is a good time to start an invasion to the surface from the Underdark and eventually take over whole Toril. Which of course is a ridiculous idea. Regardless, the dark elves start pouring into Waterdeep through the Undermountain and become the problem of your character.
The available companions in the first chapter are the same as in the OC plus Deekin the kobold bard from SoU. I skipped the OC henchmen because I do not like them and they cannot follow you into the second chapter anyway. I did not take Deekin either as -- even though he has some funny dialogue -- he still is a bard; half a party member. Besides, I had him with me on my first playthrough.
In the second chapter, I took both Valen and Nathyrra with me. Nathyrra was not the optimal henchman as she overlapped my character by being a multiclass wizard/rogue/assassin. She also seemed to have some AI script bug that caused her sometimes to stop doing anything in combat. I think it had something do with Improved Invisibility as she always acted normal whenever I told her to cast the spell on my character instead after each rest.
I replaced Nathyrra in chapter 3 with Aribeth. Who also had the same bug. The cause for that I could not figure out. It was not as frequent but annoying nonetheless.
The game's difficulty became more spiky after the first chapter. While most enemies still got gibbed quickly by sneak attacks, there were a few fights that essentially became wars of attrition. Undead mages especially were a huge pain in the butt to beat. They usually killed my companions with ease and then it was left for me to slowly wear the spellcaster down with scrolls and wands.
The chapter 2 end boss was one of such fights. I did not have any source for death magic immunity and I was constantly getting beaten by some instant kill spell thanks to low Fortitude save. At least she did not have True Seeing -- the fight would have been nigh impossible.
The final battle I skipped entirely by just purchasing the True Name of the archdevil. Cost half a million gold pieces. A pretty hefty amount, but what else would you do with the money if you not not going to fight. Rather silly, really. Planescape: Torment'sque the True Name stuff, though.
Hordes of the Underdark is fairly entertaining but playing a rogue might not be the best choice for an even experience. A caster class might be more fun, a wizard especially as they can get an extra companion in the form of a powerful shield guardian golem.