I can't comment on the radiator treatment, but I completely fixed a power steering leak in a Nissan Maxima with Stopleak. Over the course of two weeks, I'd add a little each day until I'd used about 2/3 of the 12-16oz container. My sister still drives the car and the power steering is perfect. From losing 1/4 quart per week before treatment, we have not needed to any steering fluid to the rig in over two years after adding the Stopleak.
I agree that these products are not going to fix serious issues, but they are a good first attempt at fixing a leak. If $8 worth of Stopleak had not worked on the Maxima, we easily could have spent $500+ on a steering rack.
Furthermore, if these products routinely clogged up and damaged the systems they were trying to repair, there would be lawsuits and lots of bad press.