I'm whelmed by it. (Note that I haven't read most of the Mistborn books and am likely missing context.)
I think Sanderson himself knew that this book would have severe pacing issues from all of the humans-on-Roshar backstory he wanted to cover, and so he set up the "territorial conquest up until the Contest" approach. However, this approach had problems across the board:
The backstory:
Unless it sets up some sort of forgotten doomsday device, there's no threat in flashbacks - all those events happened up to millennia ago. And not only is there no doomsday device, there's also nothing particularly noteworthy about the flashbacks. We learn (but should have already inferred, given that their ethnicities don't line up) that the Heralds are pre-humans-on-Roshar. While the final component to the Recreance supports why the Stormfather assumed humans were bad news, we already knew from book 3 on that the world wasn't strictly two sided (Odium vs Honor), with the corrupted spren, Cultivation, etc. in the margins. Beyond that, I am not sure we learn anything* in the flashbacks, and those are a significant chunk of the book.
*Well, we do learn who Shallan's mother actually is. However, this introduces a host of problems, as the Stormfather explicitly stated (in Oathbringer) both that Taln finally breaking was what started the new Desolation and that no Heralds have died since they broke the Oathpact. To justify this, Sanderson reveals that the Stormfather, the remaining chunk of Honor, in a book titled Winds & Truth, has been lying constantly. However, this doesn't make sense either in-universe or out-of-universe:
In-universe: What does the Stormfather gain from lying about this? It helps neither him nor Dalinar's forces. Hell, Jasnah explicitly theorizes that "Hey, we should go kill a Herald and buy ourselves some time" - it wouldn't have been useful information for everyone to know that this only bought six years last time around?
Out-of-universe: Again, what does this get us? Sanderson saying "Taln's the bestest evar, he didn't break!"? Or does he need Shallan to have "Can die and have hijinks in Braise**" powers in the future? Either way, we get no payoff as of now, in exchange for not being able to trust any of the history.
**Then again, why would she? Hell, in general, why would the New Oathpact work, if Dalinar broke all agreements AND Odium doesn't technically exist anymore? Do we even know what the New Oathpact actually does?
Territorial conquest:
First off, it takes forever for the fights to actually get started, and once they do, Sanderson makes it way too obvious which ones actually matter.
Shattered Plains: We know from The Sunlit Man that the Sigzil's battle goes poorly but not disastrously, and you can summarize that chunk to "Moash kills his required minor named character per book, and the independent singers deploy legalese" (that may or may not apply to Retribution).
Thaylen City: Here too we can guess there's no actual conflict far too early. And while I appreciate Jasnah getting knocked down a peg (essentially offscreen, she's gone from 'Scholar who's ahead on the Spren curve" to "Acting Queen of a country, first radiant to go fourth oath, and entirely rewriting Alethki politics (ending slavery, introducing democracy, having women in combat, etc.)", for all the time spent getting to that point, Jasnah's self-defense is laughable - for instance, I'm pretty sure she would say "Yeah, my spren and I had suspicions about my sister-in-law that tried to partner with an Unmade, was fine starving her populace, and tried killing her husband and everyone else. Why aren't you leveraging my foresight?"
Azimir: The bright spot of the book. The "reverse siege" is a neat concept, and Adolin remains the only likable lead. Yes, the Unbound are yet another deus ex machina save, but those are par for the course with this series. A bit concerned that Sanderson implicitly states "This is the modern audiences nation!" while also leaving them in the best position to be the power base of the back half.
Shinovar: While initially I was excited to learn more about the pre-Roshar civilization, its quickly made clear that they're just as delusional as to the actual historical record as anyone else. And while Sanderson can still write action well, I question whether 'Szeth's Knockoff Game of Death' was worth the page count. (With intermissions where Kal near instantly mitigates millennia of mental trauma).
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The future: Frankly, Sanderson does too good a job establishing New Odium's powers (e.g. Kharbranth). He's so powerful compared to everyone else that there had to be a complete setting reset to make it plausible for the conflict to continue into the future. While I'm interested in seeing what that new normal is, I'm also worried about the huge gap Sanderson is taking in the universe (other than the Rock novella, it sounds like we're getting the entirety of Mistborn Era 3 and Elantris books 2 and 3 before we get book 6) - just how many revisions and 'corrections' to the past storyline are we going to get when Sanderson finally swings back to this?
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TL - DR: Better than Rhythm of War (but then again, literally what books of Sanderson's aren't), but not as good as Oathbringer.