One of the best teachers I had in college taught a class that was considered quite difficult. He started by telling us that he had taken the same class at the same college. Three times. So he knew where the hard parts were, and how to help us get through them.
By contrast, some of my teachers had learned something easily that we did not, and had no idea how to help and/or thought we were just being difficult. Of course it is also possible for someone who learned something easily to help others through the hard parts.