We bought ancestry DNA tests for our three younger sons. Two are adopted from Romania and we suspect they might have different fathers, the third is a nephew we brought in when he was 13. His own mother was adopted and raised by a Jewish family. We hoped to find out more, and that is happening. Good times, lots of conversation, analysis, questioning, wincing.
They send you a list of relatives in their database, with a narrow estimate how close they are. If all you are doing is trying to get a bead on ancestral information then everyone is happy. Pretty much. If you go for a DNA test, you are already at least vaguely prepared for unpleasant surprises and where they might come from. We went into this hoping to get some clarity on previous generations. That's what most people are looking for.
But when it's a 21 year-old taking the test, and the identified "2-3rd cousin" is a 70 year old with an Italian last name, suddenly you are looking at messaging them "Hey, did you have a brother or cousin who got a Jewish girl pregnant 50 years ago?" That's likely not what he signed on for. And seeing that, and turning the telescope around, I have two brothers. I have cousins who I know a fair bit about and am fond of, but do I absolutely know whether they have children out there that I am not aware of? And do I want to know that if they do? My father was in the army of occupation in Hokkaido, and revealed significant sexual impulsivity later. So, maybe a half-sibling in Japan for me? I wouldn't mind that - I don't know how the Japanese look at that these days. At my age, and seeing the dark underside of life as a social worker for decades, I'm not going to be bothered or heavy in judgment, but gee whillikers, Skippy, if they wanted to keep this secret they may not like being helpless in the face of other relatives being able to figure this out. Bad enough that people who are looking at the record could compare wedding dates and first child's DOB*, but this takes it to a new level. I like my siblings and cousins. If they have secrets they want kept secret I want to honor that, even now, when we all would just sigh, and tell the stories and no one be angry or disdainful.
Tracy and I have straightforward genealogies (we think) on 3 of 4 grandparents each. I can tell that great-grandfather Charlie Wyman is my ancestor because we've got pictures, and he looks like me, and more like my brothers. We would be checking the DNA results largely to probe more deeply into those missing 25%'s. But doing that opens us up to other, one or two generations younger, seekers who might tell us things by asking things. Do we actually want that? The test is cheap. The results are approximate, but at the near distance of close relatives are often inescapable.
*"The first one always comes early," was the midwife's joke. In my mother's generation, an item of shame. In my generation, it was a bit embarrassing but "he made an honest woman of her." In my children's generation, there's a sort of quaint, old-fashioned honor about it. A giggle and "how sweet."