To what degree can sociology truly understand, generalize and predict social behavior?
The only possible answer here is "to some extent." Sociology can do these things to a degree, but it cannot do them as accurately or completely as it would like.
The problem with sociology is that it studies human beings. Human beings are not predictable enough to allow researchers to find absolute laws that govern human behavior. Let us look at the sociology of social movements, for example. We know that oppressed groups will, in general, rise up in a social movement when they feel that they have a chance to improve their situation. But we cannot predict when a given group will come to feel that they have such a chance. In this way, sociology can generalize about human behavior, but it cannot understand it well enough to predict it with any great degree of accuracy.