Hypothesis: a longer gap between books 2 and 3 of a trilogy, relative to books 1 and 2, is a predictor of low quality of the ending, possibly retroactively hurting the first two books, because long writing time is a sign the writer made promises they don't know how to keep.
Robin Hobb keeps writing books I kind-of-like and almost love. The stories stick in my head for years, but they never come close to being my favorites.