On my commuting bike, I had a plastic SRAM rear mech crack, and the chain wrapped itself around the cogs, jaming between the chainstays. Needed a vice to unravel the thing, and only a steel chainstay could have survived this abuse. Ive never had this problem with metal rear mechs.
Ive has a few spokes damaged when the chain comes off the large cog. A spoke protector, or a large cog with a protector would solve this.