From what I've seen with Sarri:
First, the defensive shape if we are actually forced into a low block is going to be 4-4-2. So he has to come back beside Pjanic. Now, in matches like against Bologna, it was actually really rare that we even got into that low block as our first action was basically always to press and recover the ball. But in theory, he has to come all the way back into the midfield shape.
In terms of pressing, he is pressing an area which is very hard to press, which is the middle of the field right behind our strikers. There are so many opponents that can drift in and out of that area that it's very difficult to know who to attach yourself to.
Coaches always speak of "lines of play". Well I think sometimes Berna is basically his own line of play. You have the strikers line pressing, then Berna by himself, then the CMs behind. So I think it's difficult because it's not just that he has to move vertically but also laterally, because there are not many teammates in his line of play. For example, in the line of CMs behind him, if an opponent is drifting laterally, they just get passed on among the CMs. But in Berna's line, he has to follow them across the pitch to press successfully.
I think there was a Sarri quote about how Berna arrives in attacking situations without energy. I think this is why.
The absolute pinnacle of this position was Vidal, of course, who arrived in attacking situations and just switched to his 3rd lung.