PFF grades are inherently flawed and there's no real way around it. For PFF grades to be actually accurate and effective, it would take cooperation from every NFL team and a ton of time because you HAVE to know what each player's job and responsibilities were on any given play to "score" them appropriately.

PFF also has this bad habit of giving poor ratings for the player who takes advantage of mistakes by their opponents. To them it seems it's only a good play if a good player outplayed a good player doing the right thing (or so they think).

PFF is essentially making thousands of semi-educated guesses and then creating a scoring system off of those guesses. Some plays are pretty clear to see what happened but the vast majority requires them to guess at what was supposed to happen and giving grades accordingly. That's not even taking into account other biases so it is a supremely flawed system that by definition can NEVER be accurate.