Which bolts and other fasteners should not be loosened using an impact wrench? For example, I've heard that it's not generally recommended to loosen spark plugs with an impact. Nor is it recommended to loosen head bolts using an impact.
What's the reasoning behind not using an impact to loosen these fasteners? What other fasteners should not be loosened using an impact wrench (e.g. cam sprocket bolts? crank bolts?)