Curious why you say this. I thought he was very good in the deep middle. Other than the Charger game--even Earl couldnt have gotten to those two balls in the corner of the end zone imo, and the Saints game where he was in position but misplayed the ball.
One metric I like is yards/snap--basically how many yards does a player give up in coverage divided by the number of snaps where they are in coverage--Joyner was fifth amongst Safeties, Weddle fourth. Malik Hooker was first. But watching the tape Joyner really did a good job defending deep passing game. Maybe I'm missing something.