A list for every time they tell us something is true because an undiscovered and/or isolated tribe does it:
1) How well do we know their language, including diction, implication, historical metaphor, body language, sarcasm, et cetera? How many languages and dialects do they have, and how many are they willing to use in front of us?
2) How much can we trust any given person who is acting in front of researchers eager to hand out chocolate, liquor, and tools in exchange for pleasing information to use in a PhD dissertation or to obtain a U.N. grant?
3) Despite (apparent) geographical isolation, how well do we know what groups of people may have traveled to that area in the past, and been killed or assimilated? What knowledge might those people have brought or caused to be lost?
4) Even if these tribes haven't come into contact with "first world civilization," how unlikely is it that they never noticed that other tribes in the area completely vanished over the past century or so? Did they ever speculate about why that might be?
5) Do these tribes have the abstract reasoning capability and/or the language skills to be able to accurately portray their situations to researchers, even if they are as completely honest and completely innocent as the researchers want to believe?
6) When these tribes hear aircraft passing overhead, see contrails, or see the glint of jet engines through a break in the foliage, how does that affect their worldviews? If they've been observing similar phenomenon (propellers etc.) for the past hundred years--since well before modern anthropological quarantining was established--how have those observations colored the societies that we see now?
7) In that instant of first contact, when the tribes first experience the presence of outsiders, how pristine and untouched does their worldview remain before information is exchanged?
8) Imagine a flying saucer touching down next to a dwelling in rural Arkansas, and questioning the dwelling's occupants about the history of that society's culture and science. How accurate would the answers be?
9) Have two people, from the same general ethnic background, and who have attended the same small town church ever since grade school, ever had fierce disagreements about the nature of the world, or the history and functions of the small town in which they live? Have groups of such people ever had such disagreements? What, then, makes the opinions of any individual(s) selected for study from such a tribe reliable as to the homogeneity of tribal belief?
10) If you're at work trying to get something done, and some patronizing researcher shows up and wants to ask people questions about their feelings about the company's mission statement and the employee's role within the company, who is more likely to be eager to spend time with the researcher to provide information: the more knowledgeable, capable people, or the less knowledgeable, less capable people?