[derailleurs freeze up right?]Maybe they do for some people, but not in my experience. I get the impression (perhaps incorrectly) that the people who report troubles just find one gear and stick with it for miles and miles, and then are surprised it's stuck there by icy build-up.
Probably describes me. A small touch of WD-40 every couple of days helps this enormously. Get it right on the cable housing ends.