Christopher Tolkien, 1924-2020

I feel that there is a massive difference in qualities between the books so was not surprised by the differences in the films.

How many times are authors (and fans) disappointed by their film equivalents? “Not as good as the book” is a frequent refrain but then should that be a realistic expectation? Does that go the other way, have film viewers who have gone on to read the book said it was not as good as the film?