Exactly it's like the draft grades that come out on the Monday after the draft. Rosters are not even built yet. There is no way to know once bullets start flying in September.
Not to mention the difference between success and failure is miniscule.
Think about this Ben Johnson gets too cute in his first year and plenty of his triple reverse, flee flicker - half back passes get picked off. Say that happens in a bunch of big spots. He looks like a fool and finishes the year on thin ice.
If those same plays hit at the right times he is a genius and candidate for coach of the year.
Execution on a few plays can be the difference between success and failure for a coach.