We just finished this adventure, and what a ride. It is quite difficult, we lost a ton of characters, but almost everything felt fair.
I completely disagree with the "lack of RP" there are plenty of opportunities and tons of lore for the fans of Golarion. (The end level was... just amazing, what a reveal!)
Some levels are really mean, and it will frustrate players no doubt, but if they want to be challenged, and you like playing to the will of the dice gods, they will have a blast.
We loved the characters on Fort Inevitable and it is most fun when paired with Thornkeep. We ended up doing a lot of sidequests and meeting very interesting NPCs (Dremagne, Iliara Starcloak, Oritian, Abernard are some of my favorites) *When I say interesting NPCs, I mean that they have a lot of potential, not that they come fully fledge and with 3 pages of backstory as some GMs might expect from a written adventure. (I'm fond of the simpler NPC, so I can have fun making them my own)
I was going to give a rundown of the levels, but I feel it would had too many spoilers and I liked the feeling of surprise that we had with every new step we took towards the bottom.
Number of Sessions; 26 over 1 year 1 month.
Hours of Playing; 165+
Major NPC Deaths; 2
Character Deaths; 11 (Strix Medium, Aasimar Cavalier, Gnome Inquisitor (2 times), Undine Hellknight, Fetchling Wizard, Grippli Monk, Human Armiger, Half-Orc Oracle, Aasimar Warpriest, Suli/Elf Tecnomancer)
The final party that completed the Spire were; Grippli Monk, Aasimar Warpriest, Suli/Elf Tecnomancer, Human Gunslinger)
Hardest Enemy; Lady of Thorns and Lady Estrekan of Almorain Blue
Hardest Level; 13 Throne of Azlant; for Abadar, Lady Estrekan and the Abraxas trap destroyed our group. We ended up with just 1 survivor. An incredible way to finish the Crowned Skull Quest
Best moment of the Campaign; Klanky (Controlled by our Tecnomancer) defeating the final boss of the Spire
I Completely recommend this adventure. Is not just a Superdungeon, it's an amazing story to be retold over and over.