I don't agree with the PFF ranking on Satele, he's better than 31st, he isn't a good C but he is adequate, imo he's always been an underrated C, he does have his struggles against the top flight DT/NTs, but so does everybody, he does end up scrambling at times but he quietly doesn't give up many sacks, even if he is getting bull rushed and is scrambling a bit, he has an ability to stay in front of the guy, giving the QB every second he can.
You're focusing on pass protection and yes he is an adequate pass protector. The reason he is rated so low is because he is an atrocious run blocker. You also have to take into account the number of snaps that accumulate his grade. He had 839 snaps, most of the other starting centers had 1000 or more.
If you sort the rankings by pass blocking, Satele is ranked 19th.