Using drywall screws for electrical boxes?
I used drywall screws in the past for electrical boxes and stopped using them when I found that they snap off too easy when using a powered driver to insert the screws. They can also snap if there is any reason that the electrical box can flex. This is all due to the hard tempered nature of drywall screws. 

Now I use #8 or #10 pan head stainless steel screws. If possible, when using the #10 size, I will even drill a pilot hole for the screw.