From what I have seen, PFF's methodology works very well for pass rushers, even well enough to be predictive, (studs grade high, guys with low sack totals and high pass rush grades often mean-revert, young guys earning high grades break out), and it is just awful for NTs and safeties (their 2022 "All-PFF" safety Ryan Neal got straight up released by Seattle and signed a nothing-burger contract in Tampa).
I'm thinking it's because the assignment for pass rushers is so easy to define. You go beat the man in front of you and get to the QB. For an NT, his job may just be to eat two blocks and lose slowly enough for the LB to fill. A safety's assignment can be even more nebulous. For graders who are literally guessing at defensive assignments, that has a big influence.