“Exposure to high levels of HCAs and PAHs can cause cancer in animals; however, whether such exposure causes cancer in humans is unclear.”
http://www.cancer.gov/cancertopics/factsheet/Risk/cooked-meats
Well, that is a drawback in any kind of research. Sticking with animals and cells, we can use very rigorous methodology, ensuring that each study group is identical, save for the variable under study. However, it is not ethical to study humans using that same rigorous methodology. So we are stuck with establishing carcinogenicity in animals and trying to analyze whether the same effect is true in humans without actually testing on humans. The more carefully animal or cell studies are designed, the higher the likelihood that the results will be applicable to humans.