Read Dongping Han
Accounts from well-off diaspora of any country will always be negative. It’s a self-selecting group with specific interests.
EDIT: By the way, it's not that hard either to find books written by Chinese writers not part of the diaspora that are critical of the cultural revolution (Serve the people by Yan Lianke, 3 body problem by Liu Cixin) or the great leap forward (4 books by Yan Lianke). Obviously, writers living in China that have to deal with censorship tend to be less directly critical of it compared to writers from the diaspora but that doesn't stop some criticism to shine through.a