My 71 Mustang only has a plastic fan shroud, washer reseviour, and coolant overflow as plastic underhood.
I was fixing the AC in it recently and had to remove the dash (easy). One of the AC ducts had a small area where the sun can shine through the defroster opening and could shine on it. When I touched the duct it just crumbled in that spot. Luckily parts for that 41 year old car are just about as easy to come by as they are on newer Mustangs.
All in all though the old plastics in that car have held up well. I do not think that the LS plastics will last that long.