It's always sad to see injuries seriously hamper the development of talented players. His general play was good but inconsistent and his numbers were mostly impressive for his age. For a moment there right before Barcelona bought him it looked certain for him to evolve into a special player, and Barcelona paid such a huge fee to see him realise his potential at their club. He hasn't really produced anything close to what he did at Dortmund and certainly hasn't evolved into a world class player. Martial and Rashford and a few others who struggled when he was good has surpassed him.
It is a good example to show the risk involved when trying to buy a youngster that looks like he'll become world class even before he's had time to really established himself. We did it with Martial and it paid off, but sometimes it doesn't and you are left with a player under half of the value you bought him for. It just shows it's not good to be desperate in the market.
It's actually pretty close, it's just that people's perceptions have changed based on Barcelona more than Dembélé himself. For Barcelona Dembélé registered a G+A every 114.6 minutes while for Dortmund he registered a G+A every 109.6 minutes, and the context in Barcelona is more challenging because he has never really had the opportunity to find his rhythm, Valverde has never made an effort to adapt the tactics to Dembélé's qualities.
Dembélé is very simple player, you put him on field and he will create and score goals whether he fits tactically or not. If you adapt the tactics, he will make the opposition defense look like clowns. And he is also a high risk, wasteful player who can do the most complicated thing and then next minute miss control a squared pass but he produces.