A thought on comparison of two views of FDR

I was just listening to Alan Brinkley discussing his book, Franklin Delano Roosevelt. I am also reading Frances Perkins book, The Roosevelt I Knew. Brinkley is a professor of history with a distinguished career; he was born after FDR died. Perkins was the first woman to serve in the cabinet of a U.S. president. She served on Roosevelt's cabinet in New York when he was governor as well as for the entire length of his presidency; she knew FDR personally for 35 years.

Brinkley and Perkins differ significantly about FDR's personality. Of course, knowing someone for a long time does not necessarily make one an expert on their personality, but FDR was someone so important in Perkins' life that she was likely to have studied his personality carefully. It is also the case that someone who has long experience in public life may not be entirely open about the person she served for many years immediately after his death (the book was published in 1946). Still, Perkins seem more persuasive about Roosevelt's personality and the way his mind works.

If a writer is not credible about one aspect of his analysis, how much credence should be assigned to other aspects?