Do you have a specific example in mind? Such civilizations would fall under the auspices of archaeologists, but much of the theory is still the same.

There's a difference between speaking definitively and having 100% complete knowledge. Whether you're studying whale social behavior or excavating an ancient city, you will never know everything there is to know. Why would we still have researchers? WWII was over just 70 years ago, but lord knows how many WWII scholars there are. Complete knowledge would mean the end of sciences both physical and social. An academic report is not a claim of total comprehension but a presentation of observations and the conclusions that can be drawn from them. Can we guarantee that our conclusions about ancient cultures are complete and won't be revised? By no means. But if that was enough to discredit research, we would never have begun the research in the first place. It is implicit in a quality publication that any definitive statements are only definitive in light of the presented (and cited) research.