Let's differentiate here between "stat" and "grade".
I am focusing on grade. So for example, explain the exact methodology that lands Prescott with a 71.0 grade against Denver and a 30.1 for La'el Collins.
So in other words, what constitutes say a "pressure" for a DE? Are the same standards used across each person analyzing? Without knowing the play, can the PFF understand who was actually responsible for a missed block, etc? Can PFF differentiate who screwed up on a pass play? Did Beasley run the wrong route or did Prescott screw up?
Here's an article from a Seahawks blog describing the flaws of PFF. It has it's value but it's not something people should put a ton of value in.
https://www.fieldgulls.com/2014/11/...pro-football-focus-and-a-circle-of-handshakes
About half the NFL teams pay them for their stats but there are a lot of NFL front office people and coaches who suggest it's highly flawed as well.