This is marketed as YA, probably because the protagonist, Jam, is around 15 or 16. Some have suggested this as Middle Grade, because Jam sounds much younger than your typical YA protagonist.

This is where it gets complicated.

This takes place in a police-less, prison-less Utopia (but I repeat myself), and different family structures are not looked at as different (one of the supporting characters comes from a PolyAm family).

Jam is trans and selectively mute, so you will see her sign a lot, which is signified in Italics, though she does talk when she's comfortable. Basically everyone in society is fine with that. Great!

The "this protagonist is immature" thing has a few aspects I suppose. 1. If teenagers are teenagers in YA, the adults (it's usually the adult-readers) get mad that they're acting immature. 2. We as a society do not allow black children to be children. Black girls are viewed as sexual objects much younger than white girls, and black boys are rarely if ever correctly identified by age guesstimates (by white people).

I think it's likely both of the above fit into the complaints. I will not deny that Jam acts "younger" than I would expect a 16-year-old in YA to do (and probably more than I would expect of a black protagonist). But this is a view at a possible utopia, and I think the message is that "in this utopia we try to allow children to be children"

Of course what is this book but the killing of that innocence?

The premise is that all the "monsters" of the past are gone, dead and defeated by the "Angels"

But what do you do when a creature comes to life to hunt a Monster in your town that they insist exists?

Of course, the monster exists. The problem is that everyone denies its existence.

And is there a way to deal with monsters other than execution? This book argues yes.

I think this is worth reading, but do not go in expecting a typical Young Adult title. Because it's not.