I still find that I get ingrown hairs if I am shaving the smoothest possible with at straight, fewer if my shave is not as close (duller blade, fewer passes, etc.)

Regardless, still fewer ingrown hairs than with DE and many fewer than Mach 3. I suspect the difference with the straight is that the cut of the hair tends to go straight across the hair, rather than at an angle, so the hair is not as sharp and cannot penetrate the skin on the curl back down. This is why even when I see my chin hairs, and feel my chin hairs slightly, it is less coarse than when the sharper hairs from a DE shave are sticking up.