Its supper stiff, its a composite panel which is aluminium, polymer and aluminium skin, its VERY stiff and does not move or flex at all. For the width and length of the table you hardly notice any movement and certainly a lot less then with laminated plywood or similar. I have the 5mm laminate which is just nice and strong and yes VERY light indeed.
The shelf itself is just for cooking and cutting stuff up on and i don't forsee any issues with it but i will report after this weekends trip