So I love this, fully fleshed out backstories, great artwork, any magic items associated with the NPC have their own cards written on the page (mostly) and if an NPC uses a stat block the book and the page are listed in the descriptions (again, mostly).
The only thing i can for sure criticize is some of the stat blocks created for this book have errors or inconsistencies. Mostly it's due to proficiency bonuses being miscaluclated or disregarded, like a CR 6 creature having a prof bonus of +2 instead of the +3 it should or a particular skill has a bonus that doesn't make sense when applying proficiency and the associated attribute.
There are a lot of examples of this happening, some are Ameila's attack bonus, Angel's Perception, Black Rose's attack damage, anything related to Cephelosk's INT, WIS or CHA stat, The Fissures' AC, attack and damage values and his skills, Dusan Prales proficiency bonues being +6 (CR 12), Gekals' AC and damage bonus with his staff, Malaia's infernal legacy ability doesn' have a DC, Rhizoidon's Entangle has no DC and his attack bonus is off, and Voolya's skill proficiencies.
These are just a few examples, but are moslty minor issues, and most DM's should be able to fix these themselves, but maybe the creators will update these issues in the future. Again, i love this expansion and can't wait to use a lot of these NPCs in my home game!