The huge problem with time-based qualifying (in the 100, 200 and 110H) and flights-based jumping (LJ and TJ) is how the wind can make things a travesty.
Strong tailwind for heat 1 and all the guys run 10.20 or better, including the guy with the 10.30 qualifier. Wind reverses for heat 2 and everybody runs 10.25 or slower, including a 10.15. The numbers are extreme, but done so on purpose to make clear the point. And it does happen, even "frequently."