PFF is the best place to go for stats beyond the basic ones.
There's a conceptual problem in that there's not the coach/insider knowledge to be able to fully diagnose a player's responsibilities on a given play; and not the expertise available to grade every player on every team every week even to the extent that an outsider expert/coach could do. So no fully accurate, quantifiable grade will ever be public.
Grades also seem to be affected by level of competition - like you may dominate & get a massive grade against some weenie opponent even when using trash technique, or do more vs. an All-American across from you than anyone else does to him all season, but get downgraded because he had his moments too.
But at the same time, it provides far more depth than any other public stat, and if you take it with a big grain of salt, it's helpful in generally identifying things at a high level, over time.