Was early Uruk less understood compared to earlier periods like Obaid, Halaf, Samarra and even Hassuna?

Dear fans of Mesopotamian culture

Please forgive my English, as it isn’t my first language. I’m currently reading a book by Hans (2005) named ‘From Mesopotamia to Iraq a Concise History’

In the book, he stated, the years between the end of Ubaid period to the mid of the Late Uruk period (before 3200 BCE more accurately), rates among the worst documented time of the history of the ancient Near East. Almost everywhere, remains of this period are buried beneath several meters of later deposits.

This statement made me confused. How would early Uruk be one of the most unknown periods of Near East History? Does it mean we know more about earlier periods such as Ubaid, Halaf, Samarra and Hassuna?

Thank you very much for your valuable time.