By the KTBGS (Krangthebrain grading system), Simmons got like an 80.
My grading system is similar to PFF except I give big weight to splash plays and Simmons made a few of those.
The problem with PFF is that a yeoman guy who is always in the right position could score a 80, while a guy who wasn't in the right position all of the time but made 2 big plays could score a 65. Thats why you'll routinely see guys that are only ok score high. I remember one game where Zach Allen scored really high and he didn't really do squat in a loss.
The guy that was always in the right position and did the right things should score high, no matter who he is. And the guy that made 2 big plays but screwed up the other 60 should score low.