Yeah, I don't think assists is the appropriate measure either.And it's not like Sergio's assist numbers are leaving the other fullbacks in the dust.
Sergio has 5 in all competitions. Davies has 4 and Doherty has 3. It's only really Aurier that is stuck at 1 assist for the season.
If we played more with crosses from the fullbacks I do expect that Sergio would pull ahead. But as you said we don't really do that.
What he does give us however is attacking with on the left flank far further up than Davies tends to travel. And a quicker way to transport the ball up field on the left. But when it comes to assists it really isn't that big a difference between either of our fullbacks except for Serge, who instead has shipped in with 2 goals.
The player that some people need to watch more closely in what they contribute in the final third is Davies. He's completly different to all of them, but his build-up play (once he gets past the first press, where his very poor) is actually very good. He remains in the build-up phase too if the move breaks down and recycles well, aesthetically he doesn't look good and it always looks like he's scuffed the ball when playing his signature pass inside but he's got a very good eye for finding a ball from wide to the edge of the box hit with pace (not a cross), the fact that he repeats this makes me think it's not scuffed/pulled it but what he dose (He follows these in sometimes as well).