It really depends on the plastic. Luran S, made by BASF and used by LGB for their outdoor trains from the '70s through the '90s, is exceptional. Durable, weatherproof, and UV-resistant. One of the issues though is the loss of heft. To make up for it, you would have to add weight inside the boiler. Unfortunately today, lead (Pb) is taboo to work with in industry. The manufacturers could sell the locos hollow and leave hobbyists to make weights on their own, it would probably save them a bundle on shipping costs too!
However, every time the question of plastic bodied steam locos comes up on the Forum, the consensus is that hobbyists want metal locos. Honestly with all of the failure-prone gimmicks manufacturers stuff in the boilers these days, I'm not sure there would be room to add a weight! Even the motor is almost an afterthought. 
What chagrins me is that we had this problem pretty much solved by 1940, and now it's come back. I still wonder what it would cost to cast the parts in the U.S. Except for a few precision operations (such as quartering the driving wheels), leave painting and final assembly to the end hobbyist. I can solder a few wires and plug in electrical connectors. Color match problems: gone. Most quality control problems related to assembly: gone. Make gimmicky features like smoke, whistle steam, swinging bell, etc., OPTIONAL so the rest of us can just run our trains!
"Great performance without gimmicks, it just runs better because it's made right!"
"It's the gear ratio, Brian. It always has been the gear ratio."
