Even more dangerous than these isolated courses are the schools claiming to teach self defense that have absolutely zero real-life experience, because now their students are developing horrible habits and muscle memory over an extended period of time while also increasing their confidence to unreal proportions. Some even use fancy keywords such as "Combat" or "Krav Maga" to draw in new customers (true Israeli military grade Krav Maga is a beautiful, effective hand-to-hand fighting system, unlike what is taught at most shopping center schools). More often than not, these instructors took some crash certification course themselves and now are Certified Level X Instructors in the "most advanced, deadly Marine Special Operations (wink) system of make believe combat." Some are just regurgitating the same ineffective techniques shown to them by their instructor as they were coming up through the ranks, based mostly on sport fighting techniques that have no place in real combat.
Now I must say that this article will not be which system teaches the most effective self defense. As stated previously, every system is limited by the range of the human body on how we can physically harm another person. Therefore, every system has the capability to be highly efficient in combat. It's the training methods and application of the physical techniques that determines the efficiency of what you're teaching for self defense.