Hoo boy, working through this book has been a real exercise in trying to understand what the product is vs. what it is not.
What it is is a book fleshing out several mostly previously unfleshed out species from across several pathfinder products with pages of lore, and an alien NPC stat block collection. If you understand that is what it is and want those, this is a good product. In short: This is mainly a GM-oriented product
What this isn't is a Player-oriented product. The evolutionist is a great concept that mechanically just doesn't feel like what its described as. The options for players of other classes are sparse, with 2 pages a piece for 7 classes in the character option section. limited The custom species rules are for the GM to build races, and less so for players to make custom races. The player options for the species highlighted are definitely the biggest let down. It's most often 1-3 spells and a couple feats, and thats it. None of the alternate racial adjustments and feats we've seen in other products, and only a couple of class options.
TL;DR: This is a book you get because you want the lore on some of the races deep-dived, or want a collection of useful NPC stat blocks to cut down on game prep. It is a book you should avoid if all you want is player options.
What the product is reminiscent of the 3.5 monster books: pretty lore sections, new monsters and a couple new player options. Fittingly, this one is equivalent to Libris Mortis in terms of execution: Good, but disappointing in a couple areas.
The lore and new monster sections are the best parts, making this book most useful to GMs looking to just include some undead monsters in their campaign. However, the lore of the world sections that have become my favorite part of Pathfinder products was scattershot, with several one page write ups on many regions and places rather than focusing on the most obvious places that I was hoping for more info on. (Geb gets 4 pages, the Gravelands gets 2) There's also an adventure thrown in to pad out the book, and all in all, it feels like the book is thinner than what it was intended to be.
Player options are also a slightly mixed bag, but a promising one for the future. The undead slayer archetypes are all functional and even provide interesting character ideas, as are the backgrounds.
The big new thing from a character perspective in this book are monster archetypes and races. It feels like an attempt to bring back the 3.5 ability to play as monster races. Some of them work better than others, and its clear that game balance was prioritized above other areas, as has been the credo of Pathfinder 2nd, and it leads to some weirdness. Some archetypes feel too watered down from their enemy versions and it feels like you're not playing the monster in question, but a 'lite' version of it. While this is good from a balance perspective, it leads to some immersion and apparently editorial problems. For example, the GM guidance refers to re-balancing encounter awards to take into account undead's Poison immunities; The problem is PC undead do not get poison immunities, they +1 or +2 bonuses to save against it, as with disease. Playing as a Skeleton that can be poisoned or catch a cold feels silly, especially when there's an entire section outlining that part of the interesting thing about these archetypes is how they can ignore some challenges, but are in mortal danger against other trivial things, like the most basic heal spells. Overall, I like the idea they are going for here, and hope they continue to refine the concept as it will open up more character concepts for players.
Overall, it's a solid product, especially from a providing encounters for a GM perspective, but some parts feel unfinished, and it is skippable for players unless they really want to make a character that has a concept that heavily involves the undead.