Joegr, I totally agree with everything your saying.
Thing is, that the LS's engine bay isn't very accommodating for the engine size. So it seems to me that heat can build up because its so crammed under the hood. Like, my 93' Honda accord had a 4 cyl engine and there was enough space in there for another 4 cyl engine!
Plastic parts fail, but thats not really Fords fault for using plastic. Its the manufacturer of the plastic pieces, and what grades of plastic are used. Stencil strength varies, and so does quality.
So Ford sends the word out "We need reservoirs for coolant".
Several companies will come in with their prices. Lowest price usually wins, and they get the job of building it. They're price could be so low for a reason, even tho they 'assure' Ford its top quality. And not just plastic, absolutely anything and everything that goes into the car. The Leather, the dashboard, the struts, the springs... All 3rd party companies place their 'bid' on what they can build for. Some of them, LIE about the quality they're supplying. Ford can only learn the hard way sometimes with these guys.
Results show when you constantly give the lowest bidder the job. What you wound up with is a car with failing components one after another.