Considering how many first round picks don't pan out, I guess it would be safe to say that the draft has a good amount of guessing, imperfection and luck. Including who doesn't get picked.
Since CBs spend a good amount of time running backwards, sideways, their head in the wrong direction, arms occupied, etc., do they measure that kind of stuff in the combine? Folks like to talk about 40 speed even though running flat out in shorts in a straight line focused just on running isn't the majority of football.