DH was adopted at birth in the early 60s. He hesitated for many years, but finally decided to see what he could find about about his biological parents. He signed up for Ancestry and did the DNA testing. Within just a few months, he knew who his bio parents were and gained a pretty clear picture about how he came to be. He matched with two half-sisters on his father's side, one of whom looks exactly like him (if he was an older woman!). He also matched with a nephew on his mother's side whose wife contacted DH and did some research and spoke to other family members. She suspected he would match as a half-sibling to her DH's mother and two uncles, so she asked them to get tested. Yep, they all matched. So they looked at their ages to put him in the birth order. Turns out the bio mother was married at 16, had a son, messed around and got pregnant by an older man (29) when she was 18 and still married. DH was the product of that affair. The bio father had left his wife and two very young daughters to "find himself." Apparently that meant messing around with married teenagers!
Bio mom's husband divorced her upon finding out the baby wasn't his. She went on to get married again and had another son and a daughter. Bio father is now deceased. Bio mother is in her late 70s and lives in the next city over from us. Her family approached her with the Ancestry info. She denied ever having another child. The rest of the family firmly believes DH is her son and they want to get to know him. Covid hit shortly after we learned all of this so we haven't been able to meet any of them yet.
So yes, Ancestry can open a huge can of worms! In DH's case, he wanted to open the can and finally learn about his origins. But, some people may not be ready for what they learn.