The only thing that has ever gotten stuck is a treat made especially for the Kong. I don't like them. Peanut butter, bananas, tight-fitting biscuits all get cleaned out with that long tongue, nippy teeth and lying on his back using gravity. I haven't tried the freezing method (KONGsicle).
The ASPCA has detailed instructions on the web about how to stuff a Kong. Start easy with treats that will fall out easily, then make it harder by wedging large chunks into the Kong. There are recipes like a Western KONGlette. Or how about cheese followed by your dog's food and microwave for 5 seconds. There are recipes for a Nutty Monkey and Pumpkin pie.
I haven't tried cooking for the Kong, but I can see that it could keep the human happy and out of trouble for hours. Probably longer than it takes for the dog to unstuff.
The small looks like the right size for our 14-pound dox but it is cone shaped and he eats the pointy end. From now I will only buy medium which lasts longer, and is used as a chew toy on its own.