I think that depends on how the rider mounts the bike.On that point, is there a solid reason the batteries can't actually beneficially be mounted as panniers, side mounted lowering the CoG that I believe is why technically the rack top mounting is not liked?
On a Dutch frame bike, the rear rack battery causes no problem at all because you don't have to swing your leg over the luggage rack.
If the battery is mounted on the side of the rack instead, the centre of gravity is shifted onto the side where the battery is mounted, so you gain a little on lowering the centre of gravity but losing on the equilibrium when pushing the bike.