Since this book is sort of the end of an informal series (not explaining, spoilers on lots of things) it deserves an actual review, I think.
Point: Neal Stephenson is not a science fiction writer.
Sure, he uses science fiction tropes, but he is successful when he uses them for other purposes. Snowcrash is succesful as satire, not as scifi. As scifi it's pretty much ridiculous. As satire? It's hilarious, entertaining, and a page turner.
His best books are pretty much scifi-free, and they get better the more ancient the setting. The Baroque Cycle? 10/10. Cryptonomicon? The WW2 bits are the good parts.
D.O.D.O? The good parts are the "old" parts, again.
REAMDE? Reads like ... Crichton? I say that not as a compliment.
There are exceptions, like the Mongoliad which is both historical fiction and mostly awful (except the 1st book, and honestly I don't think Stephenson wrote much outside of that one) and Seveneves which is scifi and ok.
But really, he is best writing historical fiction. And this book shows one more thing he's not:
He REALLY isn't a fantasy writer. Oh, boy is he not a fantasy writer.
Also, he should lay off the gnostics for a book or two.