This fastener could/should be better. Screw needs to come up a number on the hardness scale. #0 square head strips to easily even with the correct driver. I have a lot of experience driving s/s deck screws, finishing screws and other fasteners. My failure rate was much higher with these and the location where they strip out makes for a tedious removal job.
The advantage of the screw is that it has a #4 head and a #6 body which is very, very custom. For Flex-A-Rail it is basically a requirement to use these screws. The disadvantage is that it has a #4 head trying to drive a larger body screw. What you say is typical of what we find but they are a necessary evil. The answer is to drill the right hole size, soap the threads, and buy more than you need. When the head shows signs of stripping pull it out and start a new one.