Lovely things, precisely because they don't think normal what the locals do. And they are, sometimes, capable of quite remarkable insight. Tocqueville still gets quoted on America, and I ran across Custine because I read about it as a recommendation for understanding modern-day Russia.
They've got a few refinements on top of the usual for primary source.
The problem of misunderstandings is blatantly obvious. When there are other good primary sources about an era, it may be easier to sort them out, but still people can accept them. And when we have to rely on very few sources, or on archaeological evidence to contradict, things get lively.
Even the most honest of writers may embellish. It's not as if they expect to be contradicted by other writers on the same travels, or memory were always a faithful rendition of what really happened. And more colorful accounts always sell better.
They may also report as if it were their own experience second-hand reports. Marco Polo may never have gone to China; there is no Chinese record of him, so it's likely that at least some of his tales are exaggerated. And Julius Caesar, writing of Gaul, put in description of their lives, and such things as the Druids, but the problem is that if the Druids were as important as he made them out to be in the description, why is there absolutely no mention of them in his accounts of the war? Such important men would have been a major influence on the conflict.
Basically, you need an extra large salt shaker. 0:)