The tests are actually quite accurate. It is the interpretation that is the tricky part.
I was just poking around on Ancestry.com, and it showed me a name that it had determined is a very close relative, based on our DNA results. Yep, I think my uncle is a close relative. My cousin popped up there, too, and it placed her as actually being more distant than a first cousin--but that makes sense because she and I share a grandfather, but not a grandmother.
Another factor I look at is the ethnic profile determined by DNA. I compared my uncle, my cousin, and my ethnic background. Uncle has Bantu and Indian ancestry like I do, but my cousin (his niece) does not.
Some variations in DNA are due to the nature of the molecule. Even identical twins do not have identical DNA.
I think I probably understand the nature of the tests run by companies like Ancestry.com better than the average person. I did my Ph.D. studying DNA.
As a PhD in genetics, what would you tell someone about DNA testing that didn’t show obvious Cherokee DNA from a g-g-g grandmother that family legend said was at least part Cherokee? Does the DNA test disprove that particular family legend?