My general understanding of Ferrante's non-quadrilogical work is that it pales in comparison, and so my expectations for this book were relatively low. Not colossally low — I loved the Neapolitan Novels! — but relatively low.
And I think the book more or less bore that out. There were aspects in which it outshone those novels: I think the portrayal of youth, of budding sexuality and the sense of anagnorisis that all children go through as they learn of their parents is much more vivid and true-ringing than the childhood depicted in her more famous work.
But the book felt minor and shallow in so many respects compared to its famous siblings. Gone is the rich, beautiful, and terrifying depictions of Naples; gone is the sense of time and evolution that a reader witnesses. What's left is almost a Murakami-esque procession of plot points and themes — sure, Ferrante talks about the difference between order and justice, sure, Ferrante talks about the blinding madness of love, sure, Ferrante talks about the pain of beauty and the pain of gender — but you have heard these things before, and you do not mind hearing them again but you are not blown away by them.