It should be noted there are two flavors of drywall screw. The "metal" fine thread screw seems to do a fair job in a plastic box but the course thread "wood" screw should only be used as a last resort when the box is stripped. Like Ed says, they might just break the box.
A lot of that depends on how old the box is and how long it sat out in the sun. They start out fairly malleable but they can get brittle as glass. I found a gray "old work" box in my wife's golf cart that broke when I squeezed it. I am not sure how long it was bouncing around in the basket in the sun tho.
