Kajukenbo taught in the traditional way is effective. Kajukenbo taught in a McDojo is not effective. So, the same thing applies to Kosho Ryu Kempo. If it is taught at a traditional school, it is effective for self-defense.
But like the others have mentioned, if the student at a traditional school doesn't really push himself or train hard, it decreases the chances of him being able to defend himself in a street situation.
The question is not is kempo effective. The question should be is an individual taught in kempo effective. The answer is some are some aren't.
The art/style is only good in the hands of the person using it and the quality of the instructor.
If the person had a good teacher and they are a good student it should be very effective. If the person is not a good student and or if their instructor was not good at teaching then it is likely they will not be very effective.
jwbulldogs nailed it. Styles can't fight. People are not possessed by some unseen spirit of a martial arts style. They study the techniques and develop different level of ability to use them. If the student receives bad instruction or incorrect information on how to apply the style then his ability to become skilled at self-defense are limited by that bad instruction. So as jwbulldogs said, the question is not is the style effective, it is is the student effective.
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Definetley it is.
