Even the published genealogies can be deceptive especially those published in the late 1800s. Nearly every one seems to trace their lines back to some sort of royalty. While it is possible that every European immigrant was a descendant of a royal family line, I simply don't believe that every single one can be traced back to them.
As for messed up trees, I have a line that traces back to the Harlan family that bought land in Pennsylvania from William Penn. The original immigrant, George Harlan (or Harland), had a son named Aaron who sired a series of Aaron Harlans. One of the trees I found on Ancestry had one of the later Aarons married to his mother and as his own father.
Obviously the person who created that tree never actually read the material they were linking!
But FamilySearch.org, Ancestry.com, and other online sources can lead you to some wonderful discoveries. A wife of the first Aaron Harlan came from Mobberly, Cheshire, England. Her family traces back to some of the earliest Quakers - who kept excellent record. On Ancestry I have found scans of the original Quaker records that take the families back to the 1500s. Solid, definitive records, no speculation or creative writing!
One of my husband's lines, the de Prefontaine family, was a dead end with Peter who lived in Maryland and was supposedly from France. I accidentally searched English records on Ancestry and found the will of his half sister in England. She left property to her siblings who had moved to America. Through that will, I traced the de Prefontaines (who were actually named Papin or Pepin but were from Prefontaine) back to their immigration to England as part of the Huegenot expulsion from France. I have not yet delved into the French records but may be able to take that line back farther - if I can translate from the archaic French!