Lack of direct and corroborating evidence. Lack of specificity. IE, a very common historical bias: if it's not written down, it didn't happen. (FWIW this is

Lack of direct and corroborating evidence. Lack of specificity. IE, a very common historical bias: if it's not written down, it didn't happen. (FWIW this is a very difficult problem that I do not claim to be an expert in, but I think that's the answer to your question, unless I missed it...)