As a reader, I enjoyed the mystery element, and most certainly the setting, and things do get rather out of hand in terms of the chthonic weirdness factor – and in a way that I found rather aesthetically pleasing. I did feel that the characterisation for Andy is a little thin on the ground in terms of interiority – she's very much in the present, but there's not enough sense of how she connects with a larger world in terms of parents and friends, and her past. So it's more or less like having a first-person viewpoint for the present, which is fine, but I could have used a bit more interconnectivity and layering. This is a fault I've found with quite a fair amount of YA, that has characters existing almost in a narrative vacuum. Not a dealbreaker here as I quite enjoyed the story. I also feel that as a main character, she could have been a bit more proactive, but then again, that's also just me.
In terms of world building, this is all pretty generic on the surface mix-and-match – so nothing earth-shattering in terms of fresh takes on assorted standard mythologies. This may annoy some. Didn't blow me out of the water, but as stated earlier, I enjoyed the aesthetics, even if I'd have liked a deeper dive. It did mean, eventually that the pacing/suspensive payoff was a bit underwhelming for what was otherwise an enjoyable story.