didn’t know apple books switched from NexPress to Indigo’s. The Indigo’s originally couldn’t deliver the quality Apple liked but I heard they also didn’t like some of the aspects of the toner based printing Blurb certainly doesn’t seem to get much out of their indigo’s. The tight integration between Apple, their software, and their providers certainly seems to work, I haven’t seen many poor quality Apple books.
I know a few who have published books who have used Apple to do short run prototypes, which looked pretty good.