These statistics are misleading and sometimes worthless without the proper context.
Messi has a pass success rate of 80%, While Rafinha's is 92%, so is samuel. Does that mean Rafinha and samuel are better than messi at passing?, It's silly to even answer such a question.
When messi receives the ball, he has only one thing on his mind, And that for the ball to reach the penalty spot, He won't try to pass it backwards or sideways, Always forward, Always looking for that risky pass that'll cut the lines, Because that's what his team-mates and coach expect him to do.
Dembele, Is similar, In that his role in dortmund, Is to deliver the ball to the penalty box, If anyone watched him play, The first thing he does one once he has the ball is to run forward, Attempt to dribble anyone in front of him, Until he can make that key-pass, or through ball or cross All are risky, But that's his role in the team.
If he was to play in midfield and asked to hold the ball and dictate passes, I's sure his passing statistics would've been much higher.
He's still 20 years old, I'm sure his passing will improve with time, But i don't believe at this moment that he isn't a good passer already.