Case studies are lousy because they are not rigorous, and often have not had any meaningful result.
See also knee arthroscopy for patients with osteo arthritis - many of these surgeries were performed before a trial with a control group was possible. That trial found that people who got the sham surgery did as well as the people who got the real surgery. Case studies are lousy, often harmful, medicine.
The thing is that you read case studies with this caveat in mind, recognising that they're statistically flawed. When you don't have better information available, case studies and low n studies give some direction, some previously observed information.
Try and tell the medical fraternity that case studies should be eliminated because they hold no meaningful result and you'll be laughed out of the room.