I talk a lot about the Big 5 personality factors, and how useful they are. So now it is time to discuss the problems with the Big 5. As in so many things, I am indebted to Steve Sailer for bringing these up first.
Problem 1 is cheating. A person who knows how these traits work can present any personality they wish on a personality test, which is really more of a self-guided assessment. This limits their use for school and work purposes. I have to watch for this myself when I take the tests now, because I know what each question is getting at. In some ways, the best assessment is the first one.
Problem 2 is cross-cultural validity. Unlike IQ tests, where the psychometricians have long since figured out how to take cultural-bias out of tests, Big 5 tests ask about behavioral responses, which vary a lot between different groups. Big 5 tests seem to not even work very well comparing different states to each other in the USA, much less to other countries. This is less of a problem if the group is self-selected, like med students say, then the comparisons are likely to be much more fruitful. I think this is one reason why the Big 5 is so much less directive about career than the MBTI has been. There just aren't meaningful correlations between the usefulness of conscientiousness for both doctors and sales clerks.