It's harder for me to evaluate novellas than novels. If I felt like the characters were a bit removed, is it because the characterization was lacking, or because a novella inherently allows less space for characterization and I must adjust my expectations accordingly? If I felt like backstory of a primary antagonist seemed quite skimpy--a figure, rather than a person--was it because he was not properly set up, or because a novella inherently allows less space for backstory, and I must adjust my expectations accordingly? 

I think, in this particular case, it's the latter, and Griffith's approach worked very well to minimize these issues. The mythic feel of the tale, its Arthurian resonances, suit the more limited characterization, and justify Peretur's superhuman excellence at all things--an excellence that would become irritating in a more grounded tale, but are entirely appropriate in a folkloric hero's origin story. I really enjoyed the way Celtic folklore and Arthuriana were woven together, culminating in a quest for the grail that wasn't actually about the grail, except for where it was; and how everyone's approach to the quest revealed the complex interconnections between historical movement of peoples and cultures in early medieval Wales. Really, it's a very skillfully done work. I almost wish it were longer, allowed room to breathe more fully, given the Hild treatment. I don't generally much care for Arthurian retellings, but this one feels like we're getting an actual, historically grounded story underlying the Arthur tradition, rather than a retelling that builds further on top of it. Griffith is really good at early medieval storytelling.