Okay, this book is a four in conceit, but a two in execution and while I usually try to rate for the attempt as well as the result, the author's idea of what he was doing differs enough from what I thought he ought to be doing that I'm keeping it at a two.
This idea had so much potential! And yet...
What this book actually needed was to be written by a fanfiction author. The author, at least in my opinion, doesn't really understand what it means to rewrite a work of fiction (well, two works). There has to be some reason for the rewriting, something MORE than just "wouldn't it be fascinating if these two stories were one?" What does this book have to say? Why did it need to exist?
My specific complaints are spoiler-tagged.

Which gets me to my biggest annoyance. Kessel, in the afterword, talks about trying to change as little as possible about the two books. WHY? That's the whole point of retelling a story! When you get into transformative works, the whole point is to alter the reader's view of the story. Fundamentally, you get NOTHING out of this story that a reasonably astute reader would not get out of reading Frankenstein. He doesn't change your mind about either Victor or the Creature, he doesn't alter the events in any meaningful fashion and then he has the gall to end by having Mary Bennet hang a lampshade on the fact that HE made no changes. That's not an argument, that's a ¯\_(ツ)_/¯.
Any fanfiction author understands that the point of transformative works is to effect a transformation.
Come to think of it, one of the scholarly arguments made about Frankenstein is that it's the story of procreation without female influence. I grant that this is the story of toxic masculinity, but by introducing Mary Bennet into this story that's about the absence of women/the affective labor we associate with the feminine and then having nothing change, Kessel undermines one of the major premises of Shelley's original story.
I also hate the idea that tragedy and violence is inevitable and that the introduction of new factors can't change that (see ch. 3 of my dissertation). The message that the introduction of those who care into the narrative of toxic masculinity can't change it just bothers me--even if its a fiction, its a fiction we need.
I would forgive the whole Frankenstein thing...maybe...if Kessel had made this Mary's story. Which, first of all, would have meant having the narrative alter her rather than having her come into the narrative altered by experience and going through the narrative with little changing about her. This would have been a PERFECT opportunity to write about an unlikeable female character and give her a redemption arc but NO, she had to be likable already.
Incidentally, spending two pages making Mrs. Bennet have a flash of self-awareness is a waste of space. It's utterly out of character, goes nowhere, and--as far as I can tell--exists only because the author is fundamentally uncomfortable with the idea that Mrs. Bennet gets to stay silly and vain. Heaven forfend everyone not meet a specific standard of womanhood. It was just so obviously out of place in the narrative that I have a hard time pinning it on anything other than the author's discomfort with Mrs. Bennet.
Which gets back to Mary as a figure. She begins as a kind of Anne Elliott from Persuasion and...stays one? She doesn't change or evolve or become a better person, which is ridiculous because you could have totally done just that! It's such a waste.
On reflection, it's even worse because this absolutely COULD have been the story of fridging Victor Frankenstein to save Mary Bennet. He practically dies on an ice floe! Like it could not be better if you'd designed it. Their tragedy becomes the fulcrum on which her redemption turns and then you get an unlikeable woman AND a story with a point.
And finally, this book doesn't stop to think about what it inherits from its 19th century forebears. There is no reason that a book written in 2018 should kill off a character one day after she admits to having had sex. I find the idea of having Kitty Bennet become the bride of Frankenstein frankly hilarious and it could have gone so well. But he basically killed her for getting laid and aren't we done with that by now? Seriously, didn't that occur to anyone? It's also not justified by the plot.
I did like that the 2010s seems to be the decade of "make Henry Clerval gay," although since he also dies, I'm not sure. That's it, next book is #CreatureClerval.

I have other minor quibbles, which is that very few people can do free indirect discourse like Austen can and, when you can't it shows. I prefer it when authors don't try. There's a particular tenor to her work that is about showing through telling...Austen tells us what her characters emotions and thoughts are, but rarely does the work of interpreting them for us and most contemporary authors can't resist.
And I realize this is arbitrary, but I much prefer when authors just hand wave the science in Frankenstein rather than try to make it make sense and give Frankenstein knowledge of things 200 years ahead of his time. So that annoyed me.
Also, Victor Frankenstein remains the worst and I have no idea why he needed to be a POV character in this book. Just because he was in the original doesn't justify it here.
The book this could have been is so much better than the book that was.