Data sufficiency questions on GMAT aren't relatively harder than the Problem solving questions asked on the exam, but they are surely more deceptive. As you do well on GMAT, the questions get tougher. This is when you are likely to encounter quite a few seemingly easy but very tricky Data Sufficiency questions. If you are not careful, you could end up making many mistakes without even realizing the same. Moreover, Data Sufficiency questions test core concepts, hence if your conceptual base is not strong, you are likely struggle on these questions.