There are many advantages and disadvantages with surveys. Let me name a few of them.
The chief advantage of a survey is that you can extrapolate data from a small group of people. In other words, you do not have to interview or get information from every single person in a certain category; all you need to do is get a random sample and use that data as representative of the whole. This saves time, money, and energy.
As for the weakness, there is always a chance that your survey does not accurately reflect the group. There may be hidden faults and other variables that you did not consider. There is also the chance that your survey has a bias that skews the data. Hence, surveys must be very carefully conducted.
As long as you are aware of these points, a survey of incarcerated offenders of sexual assault can be done. What you will have to do is find people who will be willing to take the survey. Due to the nature of the crime many may not want to take a survey.