I genuinely love the concept of splitting character race into Ancestry and Culture. The core 5e rules seem to make an unspoken assumption that a character of a race will always be raised in that races culture which is not what a lot of people play, and is not reinforced in the setting. So things like all elves or dwarves being proficient in their culture's weaponry, or a Half-Orc speking Orcish even if they've never met another Orc have always felt weird.
So splitting it into ancestry and culture makes it more flexible and fits how many people actually play the game and provides more fitting mechanics for it.
However, the bad:
I'm not sure I agree with putting all ability modifiers into culture. The book posits that ancestry doesn't have any effect on your strength or anything like that which... isn't really true. Culture and training definitely affect it, but genetics and ancestry also play a part. Hormones, muscle fibre length, limb length, and point of tendon insertion all affect how strong you can be even with training for example. So I'm not totally on board with putting all the ability modifiers into Culture, and feel there should be some in ancestry and some in culture in ways that make sense.
I also feel that book fails in some of their stated targets, given that Human Ancestry gives you natural curiosity that makes your proficient in a skill and a tool of your choice, and elven ancestry gives you proficiency in perception. Which is your genetics giving you innate knowledge of skills that have to be taught, which feels like it goes against the book's core goals. If it is unacceptable to think that ancestry might give you a slight inherent edge in physical strength or dexterity why is it acceptable for ancestry to give you innate detailed skill training?
And lastly they remove sub races so all Elf culture is High Elven, all Dwarf culture is Hill Dwarven, all Gnome culture is Rock Gnome, so the cultures feel shallower and more uniform with less variety than standard 5e. (this might be fixed in their other 2 books, Custom Ancestries and Cultures, and More Ancestries and Cultures, which I'm debating buying to see if it does fix...)
All in all I think it is a great idea that this book doesn't fully hit the mark on, but is still worth trying, and I would really like to see an official WotC version of it for all playable races and cultures, with greater depth into what the cultures are like.
I do not want them to stop trying this concept - I just want to see them do it better.
[1 of 5 Stars!]