I wanted to write a review of this because the existing reviews discouraged me from reading it for four years, which I now regret very much, but also, I think, primed me to enjoy it as much as I did, which I don't regret at all. I am hoping to accomplish the latter while only discouraging people who actually won't like it.
Seraphina in part played around with the themes of logic and emotion and whether love is worth spectacularly defying the expectations of your family and culture. From there, I think it would be a reasonable guess that this book is a book about how Seraphina and Lucian's love conquers all, where you get to see them spend a lot of time kissing and agonizing over whether to tell Glisselda and then they tell the truth and get married and live happily ever after. As it turns out, this book is not that book. If that is the only sequel to Seraphina that you want to read, don't read this one. You will not be able to to twist it around in your head until it becomes that book, and you will be disappointed.
This book focuses on different characters and different themes than the first book. If it is a love story, it is one more about the communities we create for ourselves than about romantic love between Seraphina and Lucian. That is not to say that their love triangle isn't addressed at all, just that it isn't the main story line.
That said, if you like Hartman's writing style, this book was clearly written by the same person. The author who gave us the compelling characters we fell in love with in Seraphina constructed the new characters in Shadow Scale, and you may find them just as compelling. I did.
I also wanted to comment on a couple of other aspects: (a) yes, fully half this book is spent searching for the people in Seraphina's garden. That sounded boring to me and it clearly bored some people, but I was pleasantly surprised by how little fluff was actually included in those scenes. (b) The ending. The current first review ends with, "So instead, we see an ending with many beloved characters pretty miserable and without much purpose in their lives." I hope I can say without spoiling anything that this was not my interpretation in any way, shape, or form. I don't think that it is an invalid interpretation, but I think it's misleading to imply that it is the only interpretation.
TL;DR: If you really like Hartman's writing style, you may greatly enjoy this book, but don't go into it expecting it to be a fairy tale romance between Seraphina and Lucian.