An article by James Fleury and published in the newshub.co.nz web site should be required reading by genealogists who have had their DNA tested by any of the DNA testing companies. In short, it says to don’t believe what they tell you.
Perhaps a longer and more accurate answer is to always read the fine print that accompanies the test results and consider what it tells you. Unfortunately, many test recipients don’t do that.
Amongst other things, the article advises:
University of Otago professor of biological anthropology Lisa Matisoo-Smith told me these autosomal DNA ancestry tests can be “misleading” for people as she claimed there are no biological markers that can assign people to any specific population with anything even close to 100 percent accuracy.
She said while the DNA markers are real and there is a chance, given a combination of specific markers that someone’s ancestry is from a particular region, there is no way of being 100 percent certain the markers are specific to that area and claimed it is unlikely they are at all.
Matisoo-Smith said this is because the companies providing these tests have identified what they call “ancestry specific markers” and while they call these markers “specific” they are only found at a higher frequency in particular ethnic groups or regions.
There is a lot more details listed in the article, however. You can read it for yourself at: https://www.newshub.co.nz/home/opinion/2021/05/dna-ancestry-tests-are-they-telling-the-truth-that-s-questionable-genetic-anthropologist.html.