I've started taking PFF less seriously. They don't seem to understand coverage and assignment concepts. They tend to assign responsibility for a catch to whoever was closest at the time the catch was made, regardless of who was supposed to be there.
For example: Cover 3 and the safety in the middle zone jumps a hitch and go letting the slot receiver get behind him down the middle of the field. The RCB comes from 20+ yards away and catches the receiver from behind to prevent a TD. They marked it as a bad play against the corner when it was actually a great play on his part.