I will tell you one accessory where Plastic really works .....and that's the Muzzy Zero Effect rest. The mounting block and hardware is metal but the main guts of the rest are some super tough delrin type plastic with a carbon launcher rod.
I never would have thought what amounts to almost a completely plastic rest could be that tough and reliable. It's also very quiet when installed properly.
I am glad to see that the pricing came down over what it was when the rest came out.......I remember the first production ones were selling for over $100.

So there ARE a few exceptions to the rule.