This book is a giant "tell with no show", it tells you all the times that these characters are smart, competent, friends and lovers, but the book doesn't show you all of these things.
The worst sin is that these should be people capable and with a fair amount of experience, despite their young age, in crimes but all their plans are either incredibly simple or the interesting parts of the heist are glossed over or bypassed by magic of various types, and despite how basic they are, they still manage not to make one go right.
The beauty of the heist is yes, the tension of what could go wrong, but also the pleasure of seeing competent people succeed where it seems impossible.
In this book, you have neither.

The plot itself is repetitive (it's a series of fetch quests) and any "difficulty" or moment of tension is either solved by simple solutions that make you wonder if it was such a hard problem to begin with (the very dangerous Sphinxes distracted by the worst fake quarrel between fake lovers ever) or in which the only tension is that the author in the meantime puts a commercial break by shifting the pov to another character.
The puzzles and the trials/obstacles are both banal and convoluted (as in there isn't a world reason why they're as they're, outside of "aesthetic"), and the presentation is on the level I expect from a middle-grade book.
And without going into specifics, the twists in the finale are NOPE. If you need something like this to add some drama or intensity to your ending, maybe it needs some rethinking.

The characters would have potential, but thanks to the total absence of emotional descriptions (except for a few moments of dramatic backstory) they exist only as a series of unexpected habits, quirks and empty personalities. I'm told what their character would be, but such traits rarely appear in the book (such as Severin is supposed to be cold and brilliant, but where or when he behaves in such a way, it's a mystery).
Hypnos is the only one bearable (Laila at times): he's still a giant walking cliché, but at least he's an enjoyable cliché.
Most of the time, you get the distinct feeling that the cast is only there because the author wanted to write an ensemble cast story, and only after, she decided what to make with them.

The setting only pretends to be Paris in 1889: the characters act, speak and act distinctly like 21st-century people, and forged magical items are used to overcome whatever technical limitations there may have been. Or, in general, whatever plot point the author wants to solve.
The book simultaneously makes lore drop in useless moments in which such information has no reason to exist and forgets to go into any detail for any extra that could make the world more realistic and lively.

All of this is not improved by the fact that the writing style is simplistic, but also getting lost in descriptions of things unnecessary to the plot (so many pretty dresses, and pretty masks and nothing else) and then lacks the details that an internal POV, albeit in third person, should possess.
And if I see another adverb used as the only description for a tone of voice I'll scream.
Having said that, my opinion of the writing style is probably worsened by the fact that I read the book in Italian and this dry and without depth, style sounds even worse in Italian (we tend to be very verbose as you can imagine from my review)

To summarize, the entire book is pretty but empty. All tell and no show.