Machine screw threads in plastic can crumble. I used to "through-bolt" plastic mounts, but on some planes the nylock nuts are not accessible. On some mounts with the angled beams, spot facing ledges for the nuts to bear against is more trouble than it is worth.
The screws that work every time, never back out, almost never strip out, are the sheetmetal screws. The threads are deeper and not so close together [compared to a fine thread machine screw], which is a stronger arrangement in plastic.