I’m suspecting ‘not very’. Certainly nothing here (so far, I’ve only played the first two CDs) that even remotely worries say 1. Outside, Heathen, The Next Day etc. It’s not terrible, but that isn’t the bar to jump for a Bowie album.
That's pretty much my take on it. It's not bad, but it's not Bowie at his best either. Like you I also agree that a lack of context for the 3 discs is somewhat annoying. It's for hardcore fans only I think and even then it's still a bit of an outlier.