Even with modern genetic testing you could ever know only part of your ancestry and with Y-chromosome testing you could only know of one paternal line. As you trace your male fathers further and further back they become a smaller piece of your overall genetic puzzle. I can trace my Y chromosome through my father & his father but it tells me nothing of the genetics of my other 3 grandparents or their parents (i.e. my great grandparents). So I get the data about one great grandparent through my father and grandfather tracing my Y chromosome but lack information on my other 7 great grandparents and that just gets further diluted exponentially as it goes back farther into history. Still, I suppose some knowledge, if even small, is better than no knowledge but I wouldn’t put all my stock in genetics yet to identify yourself until we improve the quantity of data we can get.