The flawed second act makes the countess' peaceful retreat look like a bordello, the tippling knights disguised as nuns was well sung but unimaginatively staged, and Ory in nun's habit, wooing the countess in the dark, while unknowingly holding Isolier's hand, is turned into a tumbling threesome that doesn't come off, since it isn't at all clear which characters are aware of whom they are mis-grappling. But the first act was good enough to make up for the second.
I like alternate history when the point is to show that some of the things we take for granted weren't inevitable, but I think this story could have been told without the added conceit, by constructing a future culture that included all the elements he wanted to adopt from Aztec culture. Still, Harlan's extrapolation of Aztec society to technological imperium feels consistent and believable.