If you are using the proper riding techniques, there is no harm in using a fixie. But people do tend to get injuries, especially if they are not experienced riders. The main reason is using a gear that is too large for your ability. Larger gear puts more stress on the knees. You can use smaller gears and higher rpm to make it easier. Another reason is stopping the bike abruptly. Unlike a regular bike, you should use leg muscles to bring the bike to rest when you are riding a fixie. Otherwise, all the pressure suddenly goes to your knees. Smaller jerks for prolonged periods are also bad for the knees.