Early onset Alzheimers, and at least with Snuff, there was far too much happy Vimes, which had the effect of taking a double dose of valium. Also the "let's explore social constructivism, but with slavery" really didn't work all that well except for the reveal that Lady Sybil is arguably as powerful as Granny Weatherwax in her own way. I felt he kinda pushed his thesis to the limit there.