This class of product works in such a way as to build up around tiny passages such as leaks, thus sealing them off from leaking further. Unfortunately as you suspected, it also starts closing off thin passages such as radiator tubes, reducing cooling effectiveness.
The only real use for this product is to stop an expensive leak just before resale of a car, or as tkamiya said, to squeeze a few more miles out of a car that nobody cares about anymore.
I never have and never will be a fan of chemical fixes for a car.
95 E320 Cabriolet, 131K