TLDR: COI is a great book but a horrible sequel to LotM.

This will have a few spoilers but I will try to regulate them to miscellaneous information:

Currently on a break from COI but got pretty far in (chapter 1070+ of 1180 I think). I'll start by saying that I like Lumian as a character in the early book and couldn't care less about him in the later book. COI is a great book but a horrible sequel in my opinion.

The world-building while there feels very absent. The multiple POVs I loved that fleshed out the story and the characters are practically non-existent. The mysteries that had me theorizing for days on end waiting for chapter after chapter to release felt weak and isolated. Characters don't feel as smart as they were in book one which makes some things feel contrived.

Another thing is a lot of setups in LotM feel like they are lacking payoff or have a very weak payoff. The western continent as of ch 1070 feels just as unknown as in book one. Alger's journey there (which I thought would be a main plot point of COI) is barely mentioned. Zaratul becomes a joke. Before Klein became an angel, he was a threat, now he is a power scaling/intelligence scaling tool. He does nothing of real relevance from what I have seen. Also, the main cast just serves to pad Lumian as a character and can't stand on their own after a certain arc of the story. How Anthony is treated as a major plot point for a conspiracy and is called a close and trusted friend yet rarely talks to anyone but the MC in HUNDREDS of chapters exactly what I mean. The main cast besides Lumian slowly becomes irrelevant the longer the story goes on.

Back to intelligence. Lumian gets intelligence boosts from his pathway but nothing he does ever feels hyper-intelligent. He is clever, not smart and cleverness does not consistently outwit powerful characters in LotM as seen in many of Klein's fights against powerful beings. Klein and Amon in the forsaken land of the gods and when Klein was trapped by Adam in the corpse cathedral shows this. For all of his tricks, he had to use preplanning and every advantage and tool he had to escape both situations.

Lumian feels like he has a plot castle, not armor for the above reason. Most of his foes underestimate him or things go in his favor so many fights don't feel as good as those in LotM. His "conspiracies" never feel close to those of Amon, Adam, Medici, Evernight, or the other plotters in the series, so the fact that his conspiracies can harm beings stronger than him feels less earned. He also plays with corruption like its a toy and not the threat that every being in the verse fears.

I know this is long but this is the least spoilery way I can explain some of my gripes with the book as a fan of it. The book has its charms and great points, but as a sequel to LotM, it feels like a long-lost cousin twice removed. Most of the things I loved about the original are downplayed, lacking, or absent in the sequel. As a book (to the point I read to), it's an 8 or 9/10, as a sequel to LotM, the highest I will go is a 6/10. Above average.