no shot is hard if you master it
.
however it is certainly harder to exert force and control on a BH smash than on most other strokes. just not a good anatomical constellation to exert force. this is because the muscles that internally rotate and flex the shoulder forward are much stronger then the muscles that externally rotate and extend the shoulder back. those muscles (rotator cuff) are causing a lot of injuries in overhead throwing because they are weaker and can't handle the heavy internal rotation forces during the deceleration of the follow through.
On top of this the ROM on this motion is also limited thus basically no follow through is possible.