When you go for brain dumps, you go for a set of questions that may come in the exam. You have to cram sets of questions that in the end may not be there in the exam. Since the questions keep changing in the exam, there is no passing guarantee. But with proxy exams, there is a 100% passing guarantee since they are done by certified trainers for the same and use their knowledge for the questions making it an issue-free problem if the question changes.