Even with all metal cameras and lenses, if I have a large lens mounted, I will carry it by the lens and hold the lens with my left hand whilst using it.

Metal is not immune to breaking and with a big enough lens, could be more prone to disform permanently than polycarbonate which can move a bit then go back to its original shape.

Just use a bit of common sense in handling and you shouldn't have any problems whatever the lens mount or body is made from.