I have seen Cuisinart food processors at Goodwill... but you have to hunt and keep your eyes peeled and be patient. (Now, if I ever see a Cuisinart mixer in red and chrome at a thrift store I will die and go to heaven!!
I was happy for years with a Black & Decker model but I used it almost exclusively for hummus (we're big hummus eaters)... in fact I just bought another B&D at Goodwill that has never been used, and yes, I bought it to make hummus with.
I figure if it's under $10 it's not a big risk. I think the reason you see them in thrifts is the same reason you see juicers and espresso machines. People think they're going to use them a lot, in reality they use them 3 times the find it's too much trouble. That or folks receive them as gifts and pass them on to Goodwill. My grandma had an espresso machine in her cabinet for 5 years that she'd been given as a housewarming gift and used once. I got all gung ho about the whole juicing craze several years ago and bought a juicer but after realizing what a PITA it was to clean it up after making carrot or orange juice, I donated it to Goodwill as well.
Anyway, good luck with whatever you choose, but there's my 2 cents.