As someone looking a history I find us relatively hypocritical over what we accept as a primary source material. Some times this has to do with the paucity of material, such as in the dark ages or roman period or earlier. Some times it just seems like a selective choice.
Case in point Zebedee Coltrin is credited for having linked the idea of a the Priesthood ban to Joseph Smith as opposed to Brigham Young. Coltrin is pulling from a memory over 35 years old when he is looking at the issue in 1879. I mean how many of us can remember incidents clearly a year ago let alone 35. But at the same time you cannot devalue it out of hand.
So historians have to come along and look and compare records from the period. In the end simply make judgment calls. Like, it appears that Joseph Smith did not say any such thing in his life that was actually recorded. And what was recorded was contradictary within itself. As well Coltrin, as a Southern origin member of the church may have a bias which needs to be considered.
Yet historians will often use memoirs and after the fact interviews as primary source documents. I myself have done similar so I understand the conundrum for historians and biographers.