Can a course be considered an architect's masterpiece if it is not one of the top 3 he built? I've not heard a lot of people say the Loop is better than Pacific Dunes, Ballyneal, St. Andrews Beach, Tara Iti, Rock Creek, St. Patricks, etc. I'm not really sure what "masterpiece" means in this context, but it's hard for me to square that adjective for the Loop against several Doak courses that I would prefer to play.
That being said, I'll agree with everyone that the Loop is incredible and it's fascinating to see how the course works in both directions and how different holes play into the same greens. And I'll also generally agree with the NLU complaint that the course is unforgiving. They make the claim that "good shots are not rewarded" and I would agree with that (I know the rebuttal, I guess it wasn't a good shot then). If I had to boil my feeling down, it's that the slopes rarely seem to help a ball. They are much more likely to repel it. Compared to other rumpled courses like St. Andrews where it feels like you get to watch your ball randomly feed to the hole as much as away from the hole.
If I lived within driving distance, I'd love to visit every year. But to me, it is not Doak's masterpiece because there are a few other courses he's built I'd rather play.