Cons:
-Fatphobia abounding. All characters that are depicted as fat are evil. Step father, bully at the camp, camp director, and a monster that attack later in the book are all heavily described via their fatness and it is entirely a negative thing. There are no positive depictions of fatness. (There are two non-speaking minor characters that are depicted as "plump" - due to their lack of a speaking role & proximity to a not good character I don't think they're offsetting anything.)
-Just about every single plot point has lying, deception, or withholding somewhere in its origin. Between not explaining things or actively deceiving each other it's a little miraculous anything actually gets accomplished in this novel. 
-Subpoint to the above: adults are to be trusted without question, but it is also adults that create the problems these children are now responsible for fixing. I am not a big fan of kids carrying the weight adults should be handling. 

These two.5 issues are pretty weighty for me, particularly because this is a series of books directed at children. I don't have kids, but I wouldn't want them reading a series that codes fat as bad & solidly encourages deception & doesn't invite pushing back when an adult isn't making sense but is asking a lot of you.

-Percy Jackson is 12 years old. That's too young for the things he does & for the writing to be convincing. 

Pros:
-Grover. All of him.
-There is some clever use of Greek lore, and a few magical items that are dope.

All in all, the biggest redeeming quality of this book is how quick it was to read. I don't think I'll read anything from the series.