Providing NFL draft grades feels like walking on eggshells: You never want to be the person that gives a future Hall of Famer an F and have that live in eternity on the internet forever.
Besides, giving any team a failing grade at this juncture seems harsh (unless a team decides to take a punter or long snapper on Day 1, then all bets are off). The new player hasn't even met his team yet, let alone played with them in a regular-season game.
That's why you see no failing grades below (and mostly sunny-side-up A's or B's), as it's impossible to project any tenure as an outright failure before it has started.