TNC talks to a historian about primary documents:
All oral histories are "memory" -- retrospective accounts, in contrast with documents like wills or memos that were produced when events were actually taking place. Over time, people forget and revise, as when many Holocaust survivors began to retell their stories to match up with the narrative of Schindler's List. It is not dishonesty -- it just makes oral histories a particular type of document, which we need to note has been produced AFTER the event.
In the case of the slave narratives, we would also want to do the math -- many of the subjects who were still around to be interviewed in the 20th century were very young when they were slaves. How "accurate" are your memories of childhood? How "accurate" will they be when you are 80? What assumptions do kids make that adults don't, and vice versa?