But if you read a primary source, that’s one persom who had the opportunity to make stuff up. With a secondary source, even if the primary it’s based on is legit, there’s some other guy who wasn’t there and may either be lying to you or misinterpreting the primary source his report is based on. Each new level of isolation adds another opportunity to stack both lies and mistakes onto the data.

It’s not that you can’t go wrong with primary sources. It’s that you can go a lot wronger without them.