I think there is some validity to PFF, especially in their evaluations of one-on-one situations. Meaning I take a lot of stock in their grades for receivers and defensive backs, especially on passing plays. Even with running backs it is fairly useful in terms of yards after contact, broken tackles, etc.
However, the assignments for lineman are a lot harder to decipher if you're not the coach calling the plays and knowing who has what assignment on a given play. A guy could look like he got beat when he is just trying to cover (late) for another guy that blew an assignment. Offensive lineman by their very nature are switching guys off bc the defense is constantly trying to trick them or overload a certain part of the line via blitzing.
I trust Rivera and Gettleman's evaluation way more than a 3rd party website. Both have proven to be pretty straight up in their evaluations of guys to the media.
I honestly feel way better about the offensive line outside of LT this year than any year since probably '08 or '09. Jordan Gross really was a guy it was easy to take for granted.
As long as CJ stays healthy, I feel pretty good about the defensive line. Obviously it would be awesome to still have CJ and Hardy, but I don't want to discount what KK brings. He has quietly become of the best interior rushers in the NFL. The D line still has top 10 potential, maybe top 5 if Ealy or Big Frank step up
That being said, I do like PFF. But there's a lot more to the value of lineman (and linebackers for that matter) than what somebody watching film and not knowing the play call is going to see