A few years go, as a family researcher using on line resources to build an online tree for my family, I was shocked when I realized how bad the trees could become before 1790 (or really, the American Revolution, as we have records of military service).
I had naively thought, "genealogy is a branch of history, so methods of historical analysis I learned in middle school and high school have been applied."
But apparently not so much, although it's vastly improved over the last few years, and particularly on a collaborative site like Geni.
The first lesson I absorbed was:
"if it looks too good to be true, it probably is."
None of us can be experts on all areas of history, and in fact exploring history "through" the mechanism of building a tree has been a fabulous, delightful and absorbing task.
But we can all learn to ask questions, and then go looking for the possible answers.