At worst the guy grading a team's draft is a sportswriter (or just an internet hack) who has never done any real scouting and is just basing his "grade" on whether he likes the guys the team picked. At best it's a former GM or scout (more often in the NFL than NBA though) who has some insight but who also made plenty of their own mistakes throughout their career.
What I find really funny about the whole thing is that teams that draft well (say the Baltimore Ravens or New England Patriots in the NFL or the Spurs in the NBA) start to get the benefit of the doubt from writers doing post-draft grades. Where they might have criticize a different team that made those picks, they praise those teams because history has shown their drafts to be strong and they don't want their "grades" to be way off the mark when looking back years down the line. The whole thing is absurd.
What I find really funny about the whole thing is that teams that draft well (say the Baltimore Ravens or New England Patriots in the NFL or the Spurs in the NBA) start to get the benefit of the doubt from writers doing post-draft grades. Where they might have criticize a different team that made those picks, they praise those teams because history has shown their drafts to be strong and they don't want their "grades" to be way off the mark when looking back years down the line. The whole thing is absurd.