It's not rocket science really. You cant compare Neymar and Dembele because they have slightly different roles and have different strengths and weaknesses.
Take a look at any game of Dembele during his Dortmund stint, you will see he is get off better with the free flowing football they play. Once he gets the ball, everyone starts running, making themselves available. He was utilised for his pace alot, down the flanks and for his lethal crosses. He could also pop out of nowhere, beat the offside trap and score a tap in. The Villareal and Girona game last year does show what he is good at and must be built upon.
In Barca, what do I see? Absolutely just standing and watching. The only connection I see is Dembele and Semedo as they are both direct with it. Even with their pace it's useless; if they can create something down that flank but the others are always a step behind. He is not even utlised for his pace anymore, just too restrained in the passing back and forth motion. And judging by how Semedo is not even rated in his squad, who does he connect with?
Worst of all he is thrown as a LW this year and expect to connect with Coutinho and Alba in those tight spaces, and then act surprised when he loses the ball. He cant really take advantage of Rakitic and Messi's through balls, which is extremely lethal for his pace. He is more restricted to having bring down the ball in tight spaces and lay off to Suarez, who often fail to lead to anything dangerous at all. (Not sure why he keeps trying dummies as well)
All in all, I'm seeing a very weak role for hin and a lacklustre guidance to fully exploit his strengths.