5/10 is also way to harsh. It's not a perfect system but good players tend to correlate with positive scores and vise versa.
A great example would be Bresee being graded as one of the worst iDL against the run in the entire league. This absolutely matches the eye test as he's constantly playing with high pad level and blown off the LOS.
PFF is great in certain "vacuum's" like OL grades in true pass protection sets.
You should never take their grades as fact, hell even teams can't grade players correctly (*cough cough Zach Baun*)