Problem with hand or limb shots are as follows:
You are deviating from training- this is a problem in a high stress situation and could be a problem afterward- Courts and Superiors may ask lots of questions about WHY.
Secondly bullet may miss a smaller target like a hand or limb or pass through it, ricochet and injure an innocent person.
Based on what I have read- and I have read widely in this area, and discussed it with an ex Aust SAS soldier, the preferred course seems to be a double tap to the chest and if there is still any sign of threat (esp in case the perp is wearing body armour and perhaps a bit stunned from the double impact but not really incapacitated) a slower more carefully aimed shot to the head aiming between the top of the top lip and the bottom of the nose- which would take out the brain stem.
The ex SAS soldier stressed the reason for this being that a handgun round, particularly one firing mil spec ammo does not have the stopping power to engage in any sort of 'fancy' shooting, esp in a high stress or CQB situation.