In Europe, ordinary Jews seldom possessed surnames before 1800 and many in Russia did not acquire them until well into the 19th century. They were not obligatory in France until 1808 or Bavaria until 1813. This absence of surnames makes it extremely difficult to trace ancestry back before 1800, unless a link can be found to a rabbinical dynasty. Every male ancestor in the old country had a Hebrew name for use in shul, a Yiddish name for use at home and a secular name for use among the Poles or Russians. Then, for both genders, there was the matter of tricking the Angel of Death.
Jewish genealogy is the study of people with too few last names and too many first names.
Then there is the problem of dates. Presumably, they did not celebrate birthdays and anniversaries. It is commonplace to encounter records in which a man gives incorrect dates for his own marriage and the birth of his month-old child, but knows the time of his parents' deaths with great precision. Jewish genealogy is the study of people who don't know when anything occurred, except for the death of their parents. Finally, there is a dearth of written records.