I think Celeste herself said it best in the authors note; "Bird and Margaret's world isn't exactly our world, but it isn't not ours, either". 
I can confidently say that this book scared me. Not in the sense of horror, but in the sense of how similar the "dystopian" world was to our own world. It would be false to say that what happens in the book couldn't happen, or hasn't already happened. At the end Celeste discusses how a large part of inspiration for the book came from the missing hearts we are seeing now; the removal of migrant and indigenous children from their parents. Not to mention legislation towards a group of people deemed dangerous (Muslim ban, to name one) and a threat to free speech (McCarthyism and a resurgence of banned books). 
This book tackles a lot of heavy themes and in my opinion is by no means an easy read, but I would argue a necessary read. 
Comparing to Little Fires Everywhere, I feel as though the characters lacked some depth -- although I can't help if being kept at arms length from the characters was intentional. Celeste still writes very eloquently and manages to make the story beautiful despite its heaviness. I think it is a different direction than her other works, but I think the different direction is part of why it works so well. 
The ending of the book was very unexpected for me, although I think the ending unfolding the way it does is really important to the meaning of the book. Celeste does not aim to give us a "happy" or "fairytale" ending, instead asking us to reflect on a world where the oppressors win, where people are seemingly defeated. At the same time, she does an excellent job at adding a thin thread of hope to the story, and in turn I think a thin thread of hope about the world and our current reality. 
Overall I thought this was an exceptional and unique story that does an incredible job of drawing past, present, and future events together in a cohesive manner. Her prose throughout is beautiful and haunting -- creating a particularly moving narrative.