I did the DNA test and it was really helpful in sorting out some of the dead ends ( due to illegitimacy) in my family tree. My ggGM was illegitimate and brought up as her DMs sister but with DNA I was able to link with descendants from her stepfathers family confirming that her DM eventually married her DF but not before they had 4 children illegitimately. I think the link was via her DF’s brother so couldn’t be through maternal DNA.
With DNA you work in reverse to find common ancestors. There is a film about a geneticist who used it to to solve a cold case, set in Sweden it’s called “The Breakthrough” and explains how DNA is used to track families forward from a common ancestor. Netflix may still be screening it. It helped me understand the process.
The only revelation I found was a second cousin I didn’t know I had. I knew that my DMs cousin had had a teenage pregnancy and had had the baby adopted. I had a very vague memory of visiting the cousin with my DM and my DM explaining about the baby. But DNA threw up another second cousin ( same mother as the first) born the same year as me. Neither of my uncles knew about the first baby but were aware of the second. The two half brothers discovered each other some years ago and made the papers. I would never have made the connection with the family though.
The adoptions were forced by the family, it was the 1960s, I have a suspicion that she had a third baby because I would have been under 2 when the second was born but my memory of it is quite clear. It explains why the cousin was often quite cold around us. Her two sons would have been the same age as my DSis and I. It must have been hard watching us growing up.
I think you have to be prepared for some surprises and that it may not resolve any mysteries. But as more people add their DNA to the databank you find more distant relatives.
I had no idea that one branch of my DFs family had moved from the midlands to the north west. I now have a few matches popping up fairly local to where I now live in the midlands. And on a visit to Liverpool I was able to visit the church where my gggGPs were married.
My DH, on the other hand, can stand on our doorstep and be within 5 miles of 6 generations of his family. He is from a farming family that can be traced back to the 16th century and has an unusual surname so easy to research. He is related to lots of people locally though.