I don't believe ANY author can produce that amount of work to a consistent standard, or without a certain amount of repetition.
Steinbeck produced some real duds. Orwell's earlier novels lack the sheer genius of his last two, or the passion of Road to Wigan Pier. TP is sometimes accused of repetition in reviews, and I think there's some truth in it.
I didn't like Equal Rites, because it's clearly a transitional book between the self-contained style of CoM and the more fully realised Ankh-Morpork of the later books. I didn't like Carpe Jugulem much because it just didn't work for me, and Last Continent and Thief of Time are just too laboured and complex .. they seem to be written in an attempt to get all the themes into the available pages.
I'm not a fan of I Shall Wear Midnight because it seems to me to suffer from a syndrome, but not the one previously mentioned... Niven's Syndrome, by which the plot is contorted into increasingly unworkable shapes in an attempt to tie up a whole range of plot themes which were originally unrelated or had no logical connection.
Ringworld is brilliant, but the subsequent sequels become tangled to the point of unintelligibility. Asimov's later work tends to stray into this area as well, the originally-unrelated "Caves of Steel" "Foundation" and "I, Robot" threads becoming increasingly entangled in an attempt to produce a unified continuity between things that were originally unconnected