Loved every second and every sentence of it. 
It feels like a whole new genre, with the perfectly constructed mystery to discover along the main character, Piranesi. But never shallow, quite the opposite, with profound questions about the world and it's workings. 
After about 3/4 I worried about the ending, but it was perfect. 
The way Clarke uses language and orthography to build worlds is superb.

This series should have remained a dilogy.
I never cared much about Zélie as a protagonist, but I did care about Orïsha. Unfortunately, in this volume we barely get to see it. Instead, Adeyemi introduces two entirely new civilisations with new magic. Both remain very flat and soulless with a very boring good-evil dichotomy between New Gaīa  and Baldeírik. I would have loved to see how Orïsha deals with the aftermath of what happened in the previous installments, is reconciliation possible, and what are the struggles and chances. Instead, I was stuck in what felt like a completely different story which for some reason had the same main characters who are also so shocked by this change that they forgot their conflicts.  
Modelling a fantasy world after your own culture/a culture you're closely connected to is one thing and I think Adeyemi did reasonably well with Orïsha (more on that later), but this book also strongly shows the pitfalls when trying to it with other cultures. Don't you love it when a language that strongly resembles your own is repeatedly described as brutish? And New Gaīa ... at first I thought it was based on India, but they speak Portuguese?? It is probably inspired by Brazil, but I have to say, using a colonial language in this context is both disappointing and jaring.
So not only deals the plot of this book with completely new worlds and problems then the first two, large parts of it are also unfortunately just not good. If your nation is attacked by an enemy searching for two persons that will make him immensely powerful, sending these exact to persons to attack him is just stupid. And they do it not once, but twice! Enormous distances between the lands/continents are introduced, but then everybody seems to travel 2000km in mere hours.
The writing style is very simple, Adeyemi seems to have a wish to keep an average word count of about four words per sentence, which soon feels repetitive. I read the first volume in translation and liked it best by far, I wonder if the translator was able to improve the stylistic blandness.

I first found the Children of Orïsha series in a bookshop in Banjul, and I think that has warped my expectations. I was hoping for something based on Yoruba mythology, something African, which it is on the surface, but it is mainly a very American book; and I think as an outsider, that made it harder to relate to it and understand it. It is obsessed with skin colour in a way only somebody living in a racially diverse society can be, which doesn't make sense within the world it creates. 
I'm also more then twice the age than the target audience, so I don't want to be too harsh on the series as whole. I think I would have enjoyed it enough as a 12 year old.