Too small to have enough armour to protect the aircraft and pilot. And probably not enough fuel to keep it around long enough. The gun is a short barrelled version and that might mean that accuracy might suffer.
If they need a bomb truck, they can always use a C-130 or a Boeing 737 (which is already militarised) and fly high above the crap. Of course, if the rebels get their hands on something like a Pantsir-S1, and know how to use it, shit happens.