Not really the case, some lenses are made of plastic, others are metal. There's not one part of my Pro TL package that fits the describtion "cheap plastic", and the 200mm 2.8 APO has a construction that could in no way be described as "cheap".
I concur. Nothing in any of my medium format Mamiya kits is cheaply made. You have to abuse it severely to break it. That goes from my 645 to rb67 to C220 gear.