This is an important question, because people are complex. In other words, there is not only one way to help people. Hence, the assumptions of classical conditioning can be very reductionistic in view of the whole personality of a person. Here are two challenges.
First, like we stated above, people are complex. Therefore, they have desires, emotions, fears, and perhaps most importantly wills. Hence, to focus only on conditioning or any one technique too much is one-dimensional. More importantly, a person really changes or overcomes something when their hearts and minds change. Hence, it makes much more sense to deal with the internal matters of a person rather than only the external.
Second, the system of rewards and punishment may not be the best way to treat a person. In some ways, it reduces the dignity of a person by having a narrow framework of rewards and punishments. In the end, what separates people from all other living being on earth is a mind. Hence, an approach that deals with the cognitive aspects of a person is important.