A short summary of my impressions on Hellknight Hill, and the direction AP's are taking in general:
1) They're making goblins too cute. I imagine they want Goblins to be cleaner because they're hoping hoping for a younger and more broad 5e "critical role" audience crossover. Also, because they've made goblins a core class, they figure goblins should be more socially acceptable?
I don't like that.
Something I like about Pathfinder APs is that they haven't shied away from gritty and sometimes awful themes. I don't want a toned-down 'safe' world to play in.
Cute goblins are easily fixable though. And I have no problems making things nasty and horrific myself.
2) There are some other strange plot holes, but easy to fix. I've been seeing more of these plot holes in more recent APs (since Return of the Runelords, you can see my review of Roderick's Cove under that product.)
I wonder if Paizo has been overworked with the release of PF2, and that's making the AP's suffer. I'm holding out for the January 2020 AP. That will tell the tale on whether there is another problem other than a lack of manpower.
3) Some NPCs are ridiculously obvious and feel like they're written for 12-year old players who need to be smashed over the head with plot. Again, I'm wary that Paizo might be trying to make their stories more accessible, and by doing that, losing some of the great (adult) atmosphere they've created in the past.
CONCLUSION: This book makes me wonder: will they ever do a horror AP again? If they've made goblins into friends, are they going to tone down the horror of incestuous ogres, etc?
I wouldn't call it a classic AP book by any means, but the format is good and some aspects are nicely thought out. I think it could be a really interesting campaign, but I'm very wary of the 'safe' and almost trite tone that they're taking, and of the weak plot holes that should be fixed with more time and editing.
I was worried that they would reformat the books to reduce content, but there is plenty of content in this book. It actually feels more dense than previous APs.
The underlying structure of this story is not great, and the writing doesn't help. For the first book in an Adventure Path, this one leaves me underwhelmed and slightly nervous to see what follows. It seems like a rough first draft, and it gives me the same impression of Ultimate Wilderness, where not enough time was taken to craft a publication that is worth distributing. I hope it was just a one-time thing, and that the next books will be back up to the usual standard of quality for APs.
I find that the AP books are increasingly filled with great ideas, and the quality has been top-notch for quite a while. When I read this book, I was constantly thinking about how I'd have to re-work things to make the story coherent, or thinking of details I'd have to add where none are provided in the book.
The text (room descriptions, faction descriptions, NPC description) gives very little fuel for the imagination. They largely focus on back-story that is irrelevant to what the PCs might encounter.
For example, one room takes an entire paragraph to describe where the long-dead family sat in this room when they ate their meals. That does nothing for me as a DM. Instead, why not say that the following can be found in the room: "A child's toy; a carved drawing on the table. An old bracket on the window where a telescope used to be. A hand-crafted high chair for the youngest child that's now rotting." Something that the characters could actually discover. I find myself continually frustrated by the unusable text in the book that makes it a chore to read.
Some room descriptions simply repeat what's obvious on the map (and also given in the read aloud text): "This hallway stretches from the front door and curves at a right angle before ending. A set of rickety stairs ascends to the house's second story." This is obvious from the map, and to make it worse, the read-aloud also says: "A set of stairs rises to the next floor, and the hallway turns sharply to the north past the staircase." None of this adds anything I couldn't just tell the players from looking at the map.
There are four different groups involved in this book, and the relationships between them are over-complicated and unnecessary. Several of the groups are hard to realistically imagine living in the town. The groups (and all areas) also feel static, like they're waiting for the PCs to arrive to interact with them. The descriptions of the groups are insufficient to determine more dynamic interactions and motivations, and these brief descriptions sometime contradict the way they're encountered.
For example, there is a group of "frustrated workers and struggling individuals from the lower classes who hope to make a better place for themselves in Roderic's Cove - a sort of informal and rough-around-the edges labourer's guild." This sounds very realistic and compelling to me. However, the group is named the "Horned Fangs", and their members seem to be hanging out in a dungeon, waiting to attack. All the verisimilitude is lost, as is the potential complexity of dealing with co-opted members of the town.
Another group is led by an old witch-like oracle who seems completely out of place with the rest of her members, and there is literally zero description about her motivations, or why she leads the group.
The macguffin in the book could provide a great lead-in to the dangers of the following books, but everything is strangely capped off at the end of the book. The only motivation the PCs have at the end is to follow an NPC's advice to research the macguffin they found. I can think of a number of ways this could have been done better. (The macguffin also uses a power at the start of the story that is not usable by PCs. There is some description of why this happens, but it will never be obvious to the PCs, and is potentially confusing).
TL:DR - This book doesn't match up to the usual quality of PF AP's. I'm writing this to express my concern, and my hope that the next books in this AP match what we expect in terms of cohesive and compelling writing, and as a tool for GM's to create great stories from.