My first reaction to seeing this thread was ... well there isn't an emoji for it on this site, LOL. Plastic??? WTH ????
After a bit of google and reading, seems everyone does it .. and has been for years - even huge diesel trucks have plastic valve covers and oil pans. They claim that they are 'better' because they are lighter, and can be molded as one solid piece to replace 3-4 multi piece systems. Other benefits include parts consolidation, cost savings, durability, reduced warping, corrosion resistance, more sound dampening, reduced weight, and improved fuel economy.
This from an article in a C.A.F.E. report in 2010: "It’s no longer unusual to open a hood and find plastic air-intake manifolds, valve covers, fuel rails, water pump housings, HVAC systems, brake fluid reservoirs, electronic throttle controls, engine covers, filtration systems, and other powertrain parts".
So I reckon if it's OK for a dump truck engine, it's probably no worry for a car engine.