The Genealogical Standard of Proof (GSP) is not an altar at which we should all fall and prostrate ourselves. Nothing hurts scholarship more than blind obedience without understanding. Yet two very influential bloggers used it as an answer for why ancient lineages might be suspect. I understand their point and what they were aiming for. However, they both came off as dismissive of the prospect that you can even have medieval ancestry you can prove via GSP (and you can) or about how certain aspects of genealogy actually works.
I had an argument years ago online with someone about Thomas Jefferson and Sally Hemings shortly after the DNA testing proved that one of the Eston Hemings descendants had the same Y-DNA as the Jefferson males. I noted that Madison Hemings in a newspaper interview had named Jefferson as his father based on his mother's own testimony. I said that, in and of itself, should be considered primary evidence. If a white diarist had noted his parent's names we would accept that. However, in this case, it was certainly due to the race of the individual that this testimony was not accepted as such. In that light how do you use the Genealogical Standard of Proof for communities or cultures where the production of documents is not paramount or has been radically impaired? Do African-Americans, Native Americans, and European Jews get to do genealogy or do we say sorry, the Genealogical Standard of Proof hasn't been met. You have to stop after four or five generations. No more for you.
Likewise, can you use the Genealogical Standard of Proof to prove that Julius Caesar existed? If you run down source after source, you realize that you only have a few contemporary [that is contemporary to Julius Caesar] historians that say there was such a man. Even his own written works are only attributed to him, because a contemporary person says so. Everything we know about Julius Caesar is, in fact, hearsay. But, we do accept his existence. We evaluate those historians and note when they wrote their histories. We come to the conclusion that these secondary sources are, on the whole, reliable.
This is how medieval genealogy is pursued either partially or completely depending on the date. Certainly we still have primary records from the period of 1100 and onwards (such as the Close Rolls, Pipe Rolls, Fine Rolls, etc.) and other church muniments and records. However, medieval genealogy relies upon the same factors as history in order to make a case. You can use the GSP to prove English origins and even generations back to the year 1450 (-ish). Before that you need to alter the approach to allow for secondary sources to be used.
So before all such lines are dismissed you need to do some research. I keep a list of bogus royal lines as well as those I know I can prove. As for ancient lineages, I don't enter any into my database. I made my line (entirely arbitrary to me) to be William The Conqueror (William I) 1027-1087. I include lines up to the 11th century but not before it (even though I could reasonably use Charlemagne as the stopping point). It's a thousand years ago, which seems like a nice round number. Someday when I retire and I can do more investigation, I'll add more to this. I know of one scholar, Christian Settipani, who does credible research into Gallic Roman families and has some that push back prior to 500 C.E.