I remember in the Falklands we had some old Leyland buses. The fitters had one they used to replace any that broke down and put all bits and bobs in the boot which was large.
One day it got too much and driving down the road the boot broke scattering it's contains on the road.
That night in military bar we heard it being talked about.
"Did you see that LMA bus this morning. The engine blown up it still carried on down the road with pistons and everything left on the road"
I don't think a car boot will fail in same way but it is likely to get damaged with hard items carried in it. If nothing else paint damaged. I think few of us read the car manual but they do give as already said max weights.
In my Vauxhall it states that mobile phones should not be used without external aerials being fitted. You try telling a passenger not to use mobile in your car. It's all right I'm not driving is reply you get.
So next time you see a passenger with head out of the window while using phone you will know why!!!!!!!!!!!!!!!!!!