The proliferation of online family trees makes finding examples of extended pedigrees rather simple. Here is a screenshot of one of my family lines as shown on the FamilySearch.org Family Tree.
This image illustrates the first and one of the most basic issues with these extended pedigrees. In this case, the last ancestor showing here is John Sheldon, b. 1494, d. 1572. He was supposed to be born in Bakewell, Derbyshire, England. Here is a summary of the records available for the Bakewell parish. This screenshot came from the Derbyshire Record Office and Derby Diocesan Record Office.
According to this, the earliest records for baptisms in Bakewell date after 1614, more than a hundred years after the John Sheldon person was supposed to be born. Unfortunately, there are no sources in the Family Tree to help us understand where this information originated. In fact, if I keep clicking back on this line, it finally ends with Billung, b. 780. Most of this information may have come from the following books:
Bartlett, J. Gardner. The Sheldons of Bakewell, Derbyshire, England And Isaac Sheldon of New England. Boston, Mass: [s.n.], 1926.
Even if we were to assume the pedigree going back from Isaac Sheldon, b. 1629, d. 1708 was correct, we would still have establish that one of my verified ancestor was his documented descendant. I would not be anxious to accept the information published in the two books without doing some research to determine some level of accuracy.
But the point here is that I do not have to go back nearly so far to find the weakest link in this pedigree. Examining the pedigree much closer to the present day, I see the following:
Here we see the breakdown. John Sheldon is not identified. There is no birth information and nothing showing that he was the son of Godfrey Sheldon, b. 1599, d. 1670 in Bakewell, Derbyshire, England.
Here is what is shown for John Sheldon:
This detail section of the Family Tree shows eleven possible names or identities for John Sheldon. The record for the individual recorded as his father, Godfrey Sheldon, indicates that he died in 1670 in Scarborough, Cumberland, Maine, whereas our John Sheldon, was supposed to have lived in Rhode Island. In addition, John Sheldon is listed as the 13th child of Godfrey Sheldon and Alice Frost. This couples children are variously listed as being born in England, Massachusetts, Maine and Rhode Island.
Before I would accept any of the entries in this pedigree beyond John Sheldon (if I would go that far), I would have to see some document or record showing that John Sheldon and Godfrey Sheldon were father and son.
This example from my pedigree in the Family Tree is an example of the lack of specific information connecting each generation of an online family tree. There are other, even more troublesome, relationships in the Family Tree for this particular line. I would suggest that if you find yourself relying on this type of ancestral line, that you begin the process of verifying every generational link from the present back. I frequently find gaps and unreliable conclusions when I examine family trees in this fashion.