The main problem is that the adventure is a series of encounters/things that happen, and not a story. There isn't a central conflict: it's just "some things that happened on the way to the underdark"
The best bit is the 1st sequence: "Party should get past the gatehouse full of guards, somehow". But don't do it the "official" way unless you're a party of nothing but cha/dex scoundrels of some type ... otherwise the charisma check DCs are too high. Don't do it the murder-hobo way unless you WANT to run long, because 40 v. 7 is just going to take a really really long time, even if you can win (and the right party can, easily. The wrong party will struggle). The freedom is excellent, but the adventure doesn't help the DM understand the conflicts, and doesn't help the party find various approaches that would work.
After that, it's a railroad of encounters, none of which seem to have much significance, but many of which will be long and/or hard. Until the end, when there's a scripted way of making the last combat a bit easier, but my group was too tired at that point to think anymore.
Really, the 1st part could have been expanded slightly into a 4 hour mod (investigate the leadership of the gate-house, RP heavy. Thematically focusing on the Hillspar local political issues) and in XP budget with only minor re-giggering. And the 2nd part could be a separate adventure (again, expanded a bit) focusing on the demons-in-the-underdark storyline. Jammed into one module, it loses the ability to focus on either story, and gains nothing by combining them.