"Just endings for various villains"? Theyre the the main adventure locations for four powerful villainous organisations, each fully detailed and stocked, very various hooks for involving them in a waterdeep based campaign.Honestly, it's good that you found it that way, but it did not feel like that at all for us. Maybe my expectations were too high, but the NPCs in particular felt wrong, all these high level NPCs dealing with imcompetent adventurers, etc.
Calling it "just one chapter" is very misleading. In term of pages, it's more than half the pages of adventures, since chapters 5/6/7/8 are just the endings for various villains, and chapters 1-3 are very short. It's also really the core of the adventure and incredibly frustrating if played as written, basically it's automatic movement through 10 locations with what you do there not mattering at all except if you fail.
There are a few funny things here and there, but honestly, but I never felt really engaged by the introduction either. Some DMs have apparently made it much better, but it's also probably a question of choosing the right villain. I think the Cassalanters are supposed to be the best, we had Jarlaxle and it was really not interesting, we could not understand what was the point of adversaries who were infinitely better than us and undercut us at every turn whatever we did.
In any case, the principle of the book is stupid: why have 4 different stories and villains ? The replayability is nil anyway with the same group, and as a DM of a campaign, the replayability really should depend on the players and their actions. Here, so much time is wasted with things that will never be played twice, and even less four times...
Half the adventure? Thats one interpretation I guess. Its 29 pages out of a 169 pages of adventure (yes I do count the villain lairs as part of the adventure). Even then, within those 29 pages you still get 10 city based locations and four different approaches to dealing with them.
IF you aproach it like a typical adventure with PC's clearing dungeons and killing everything in sight then yes the NPCs arent going to work. However if you see it as high level NPCs that are powerful enough not to have to kill everything and can actually work with the PCs or involve them in their plots/play different power groups off against each other then they work fine. Manshoon should not be having a one on one fight with the PCs.
Dragon Heist is not a novice adventurers campaign. It can set up a very rewarding Dungeon of the Mad Mage campaign where elements can be used over again and again. Its anything but a railroad though.