Just because a document says that somebody is “Fred Smith” does not mean that Smith is their actual biological surname, it can be an Adoptive surname.
To prove the point, a yDNA test only goes from male to male to male and that should follow with each generation having the same surname. Wrong, in reality there are hundreds if not thousands of surname variations in the match list. I know I took a yDNA test.
DNA testing does help but it can unearth secrets and throw research out of the door and into the trash. Again, voice of experience speaking.
Good luck finding valid solutions.