A heavy, dense load may hang nearer the vertical whereas a lighter load or one that has properties that may cause it to generate lift may trail somewhat more.
Depending on the drag of the load a much greater nose down attitude may be required to overcome the drag (trail angle) of the load. This may exceed forward cyclic design criteria.
A heavy load, although hanging nearer the vertical may generate large pendular forces at low speeds.
It is correct that with a wider variety of loads to carry the military has an organisation (JATE) that conducts trials in this area. It is however an expensive business.
Generally anything in a net has a 'blanket' clearance but it doesn't absolve the crew from having to assess the handling properties of the load themselves as to whether it is feasible.
A blanket VNE must be a way of giving a clearance without requiring every load to be individually cleared at great expense to the manufacturer. How could they ever assess everything that may be lifted I guess
HEDP