(a) There's no crock pot I can think of that controls the temperature of what's in the pot; in other words, fill the pot with water, and there's no crock pot that keeps the water at 130f.

(b) Crock pots are (allegedly) hard to control with PID controllers (that's why everyone uses rice cookers); the "low" setting on the crock pot is apparently still too hot.

It's also getting harder and harder to find crock pots that don't have digital controls, which is a dealbreaker if you want to use a cheap PID controller.