There are different scientific approaches used by different companies for DNA testing, and quite naturally they deliver somewhat different results.
However, all DNA testing depends on results being added to growing databases, and accuracy is only as good as the separate databases. Considering how little DNA is actually transferred from relatives many generations back, much of the "results" of such testing today are still estimates or partially educated guesses based on the barest of information. As databases grow, so will accuracy.
As to results leading to living relatives out to the multi-level cousin arena, they can be surprisingly accurate because the DNA is relatively dense at these levels. However, they are still statistically determined and can often be correct in identifying the person as a relative, but not to the proper degree.
This is still an emerging science, and tests are changing and becoming better. Older tests still in a database may actually decrease accuracy in some respects.
My wife has taken three DNA tests from two different companies with different testing methods, and none have returned any information. Either she has no DNA, or science has not found a way to read hers through the current available testing methods. That's too bad, because she has always hoped her mother slept with the postman.
I had a daughter-in-law who was at the top of the Ancestry DNA administration. She was a brilliant scientific person and extremely competent geneticist. Her explanations always went over my head.
I have also attended three Salt Lake Roots Tech genealogy conferences. At each I attended at least one DNA seminar. As much as presenters tried to simplify what and how they did their jobs, and how to interpret (or not interpret) results, my best efforts at understanding it all are written above. It's still mostly magic to me.
I have genealogy lines directly back through England to France before 1066. Ancestry cannot identify France as a region from which my ancestors came. On another line, I have direct German ancestry back to the 1400's. It's also missing from DNA results from two different companies.
They do show a heavy influence from Scandinavia, so I guess my earliest English ancestors, whom I have yet to identify, might have been Vikings.
This I do know: we are all one family, descended from Adam and Eve. The links among all generations of families are there. We are just not yet well educated enough, scientifically and spiritually, to be able to discover all those links. Yet they are recorded and known, so it will all work out in the end.
