I've just put in for DNA testing looking for family within 5 generations expecting to confirm Irish connections. The company contacts matching people giving them my name and e-mail and I get their information.
After looking at Irish on my mother's father's side for almost 20 years, it dawned on me I knew almost nothing of my mother's mother Mabel's family. Mabel Jane Kyle was dau of Alexander and Margaret Boyle (Aitken) Kyle and Jane would be her aunt. I didn't want to be completely caught out if I were contacted by a NZ only a few generations distant. My Gt aunt Dorothy Kyle was single and visited us once in Massachusetts and my aunt knew gt uncle Alexander & family. Gt Aunt Eleanor died during the ~1921 flu when visiting Mabel in (I think) Palmyra, NY. So that leaves Margaret Kyle's, Jane Aitken's, and a Clunie's families as potential surprises.
Typically I follow family into the early 1900's as most people have some sort of idea who their grandparents were. If they mention them on-line, then I often contact them. Otherwise, no. Most of my near and distant relatives want privacy.
I don't know much about DNA testing to know what sort of results to expect. It is obviously for people who want to make contact with relatives and this company's database isn't the largest. I shall see what happens.
