As someone who has been doing sports stats at the HS and College level since the 1960's While I'm glad that tools like PFF exist, they may not have enough sensitivity to accurately reflect how some players impact the game.
I find it difficult to believe for example; that Josh, who they have graded in the 90's in each of the past 4 years, has suddenly dropped down to the #6 QB in their rankings behind Lamar, Burrow, Daniels, Herbert, and Carr, none of whom play for a team with a better record than the Bills.
He has 1 game this season [KC] that is at the level he has averaged for each of the past 4 seasons. Is Josh playing worse, not that I can see, but his grades are not reflective of how well he is playing or his leadership.
Last night his PFF grade was 80.7 even tho he posted a nearly perfect passer rating, and scored a TD on the ground and threw for two more, including one to himself on the hook and lateral with Coleman.
I think their scale on grading plays may not be sensitive enough to properly credit extraordinary plays such as the run vs. KC or the Allen to Coleman to Allen play last night. I also don't think they weight game changing plays heavily enough, for example the Rapp forced fumble which IMHO turned the entire game into a rout.