Numerous observations of a representative sample need to take place in order to generalize the findings. This is especially difficult when looking at a particular group. Many groups posses unique characteristics which make them interesting studies. Hence, often such findings are not strong in external validity. The main problem with unobtrusive measures, however, is ethical.
Issues involving informed consent and invasion of privacy are paramount here.
An institutional review board may frown upon your study if it is not really necessary for you not to inform your subjects. This can be anything to vandalism to garbage. The University of Arizona Garbage Project one of the most well-known trace studies. Anthropologists and students dug through household garbage to find out about such things as food preferences, waste behavior, and alcohol consumption. Again, remember, that in unobtrusive research individuals do not know they are being studied.