Rex Mundi is a mixed bag for me. I like the gnosticism aspects of the story a lot. That sort of revisionist history, based on the actual gnostics of the first centuries of Christianity, is fascinating to me, and Nelson and Eric J do a good job of incorporating it in the story. Equally, they work the politics of the world very well, even if I don't accept how they got to that point in history and technology. Right up through the point where Dr Tournon and the Duke are using magic I'd say it's a worthwhile read, but I'm more on the fence with those new turns.
Finally, I have to comment on the Dark Horse production values. Two of the three trades have pitiful binding. One of those two has such bad binding that pages are coming out in pairs. The other at least has fled its binding in one unit, but trying to keep the pairs of pages in the book in a neat fashion is nearly impossible. And, truth be told, the binding on the tird isn't much better.