I think there is a penalty on your To Hit chance when making a called shot.

What is the penalty when you try to make a Called Shot on the enemy's Head?
What is the penalty when you try to make a Called Shot on the enemy's Arms or Legs?

Maybe To Hit chance drops by 40-50% or so? I hope we can find out the exact numbers.

It varies depending on your training in the relevant combat skill. For example, a Firearms Master has a 2/3 modifier for called shots (with a gun). With less training the penalty is greater. The exact numbers are in the manual, I think.