I've had more than one mechanic tell me this as I've replaced disintegrated plastic engine components. Mileage standards often boil down to weight, weight, weight. Many things that used to be brass, steel, and cast zinc are now plastic. Every ounce counts. They work well enough for a while, and they use the best plastics they can get, but plastic isn't metal and never will be. I suppose there's a plus side if you're talking corrosion issues. Weight is also why your spare tire is the size of an Oreo cookie.

My Eurovan's coolant system has a lot of plastic in it, and it gave me a lot of trouble in the van's later years. The stuff would just crumble. A lot of junctions have been replaced by brass kludge fittings, but I just wrote off the heater coil fittings and went without front end heat. I replaced the main coolant reservoir just before it started falling apart.