Before I ask, I love this movie and have seen it many times. I even own the poster and tape, somewhere. But...
Wouldn't an ordinary household disposal jam after something like a human hand was caught in it? I can't drop a spoon down mine without having to reset it. A meat grinder, maybe, but unless the 70's had no safety laws, than I can't see it doing much damage.
A plumber from the '70s could answer this question best.
Maybe back then men were men and disposals were disposals -- no one gave a crap about safety, right?
Even then, a hand cut up by a garbage disposal would HAVE to be amputated because of contact with a highly contaminated bacteria breeding ground. All the antibiotics in the world don't mean nothing without excellent sanitation!
Spoiler alert for them spoil sports out there! Y'all like spoiled milk, stop crying over it!