The good: a smoothly written alternate history. You can tell Moles has done his homework, on Pure Land Buddhism, medieval Christianity, and Moorish Spain, and even better, he presents just enough information to ground the reader without bogging down the story. The world is distinctive in its mix of technologies, combining plastics, blunderbusses, and ignorance of atomic weapons.
The bad: protagonist Nakada is generally unlikable. Some of this may be poor choices by Moles--e.g., we see Nakada's heroin addiction before we get any sense of personal demons that might justify it--but she also is presented as abandoning her husband and son. Her actions in the river village sealed my dislike for her (more below).
The Espirito Santo event is also fundamentally unbelievable. The nearest analogy would be, in our world, atomic weapons being invented by North Vietnam and used to destroy Saigon.
The thought-provoking: as I listened deeper into the audiobook version, I grew more convinced that Moles is a closeted reactionary. ("Closeted" because his personal website has standard left-liberal SFWAn goodthink posts, e.g. sexual harrassment is bad and Theodore Beale is a racist).
Some aspects of the story seem very bog-standard progressive, i.e., Japan's only international presence is a Relief Ministry staffed on sex-egalitarian lines. (In our world, it took getting multiple cities destroyed by incendiaries and atomic bombs, and cultural reengineering by Americans with Ivy League Ph.D.s, to create a Japan that would do what Moles' Japan does).
But against this goodthink political correctness, flashes of reaction come through. Nakada writes in her journal that humanitarian aid only serves to prolong war (an insight previously given by Edward Luttwak in Strategy: The Logic of War and Peace, Revised and Enlarged Edition). Nakada also writes in her journal that the Relief Ministry wants employees capable of self-destructive self-sacrifice (shh, don't tell a recent college grad considering enlistment in Teach for America). When Nakada veers into idealism, against the pragmatism of the ambulance boat pilot, does Moles mean for us to lose sympathy for her? Are Nakada's idealism, and the unlikelihood of the Relief Ministry, targets of satire?
More broadly, the journey up the Mississippi watershed is a journey through, in Eric Voegelin's concept from The Ecumenic Age (Order and History, Volume 4), an ecumene, a fought-over spiritual wasteland, where the refugees of smashed traditions wander in hopes of finding, or building, a new one.
Even more reactionary is that, given the time of divergence between Moles's world and ours, northwestern Europeans are essentially absent from his world. (Most parts of the world get name-checked, except England and Germany). The conquerors and crony capitalists of Moles's world, the smashers of traditions, are Muslim or Catholic. It's as if Moles is telling writers who want to have non-white villains in their stories to set those stories in a world without (significant) white people.
No telling if Moles thought of any of this, but the fact this novella, despite its flaws, got so sizable a review from me suggests there is a lot here.