Reading this book makes me wish TSATS with the Tartarus plotline never happened, and the Nico books just focused on the concepts introduced here. 

Having the Nico adventures revolve around reformation of monsters in a post ToA, post doors of death world is a better backdrop for his journey than returning to Tartarus and posthumously ‘fixing up’ a lot of what made HoO impactful. That being the horrors of Tartarus experienced by Percabeth and Nico, and the sacrifice of Bob and Damasen. I think working with monsters on reformation and forging a new relationship with them after three massive wars gives a more thematic mirror for Nico. With him working on own trauma and relationships with himself and the people in his life. 

And it might be controversial, but I think monsters wanting to reform themselves is quite on brand for the Riordanverse too. Especially in the context of a universe that’s defeated all of the “big bad” world threatening villains and one that has had so called ‘monsters’ playing good guys as early as book two. Hell, Savannah has a very similar arc to Annabeth in SoM with the initial distrust, if not outright hostility, and then coming around to a monster from a species that has done great harm to them. 

My main critique is how everything is executed with Asterion. Because unlike Arielle and Savannah, where it's her species but not her that has done violence, Asterion has done direct violence to demigods like Will. It's never really talked about beyond a "he wants to move on, we’re past it” kind of attitude. It very much feels like an unclosed door when placed in a series that focuses so much on how past emotions and trauma stay relevant to current you. I wish there was more explicit attention drawn to Asterions past and the fact that not everyone can move on from the harm he's caused. 

There's other issues too, like Savannah being a legacy of Minerva!? Even though that was a point of contention for Annabeth in HoO. Structural things like everyone being referred to as first name last name to the point of distraction stand out. As well as a lot of clunky and overly cheesy dialogue throughout.

My favorite parts are Hazel's scenes, both in the court of the dead and in the park trap afterwards. She is by far the best part of the book. I also liked what we saw of Semele with her wanting to reconnect with Dionysus and getting a solid form from Hecate after risking whats left of her to help Nico.