Luke Hughes has a better passing/puckhandling combination than Owen Power or Simon Edvinsson, as well as being the far-and-away best skater of the three. Though Edvinsson's puckhandling is very close to Hughes, his passing/vision his not even in the same stratosphere. Though Power passes very well and far better than Edvinsson as well, his puckhandling is not comparable to either of the other two.
So, why is Luke Hughes generally ranked behind Power and equivalent to Edvinsson? I would say that comes down quite clearly to their defense and play without the puck. He's not even close to the other two defensively. Though Power would benefit from another year of amateur hockey, he could play passable defense as a third-pairing guy in the NHL right now. Meanwhile, Edvinsson plays physical, high-compete defense, and although he has his warts, this quality combined with his speed and immense wing-span make him a constant force in the defensive zone. Luke Hughes is admittedly very problematic in the defensive zone -- as well as prone to turnovers which lead to high-danger chances against.
The hope is that Luke will learn to pick his spots better to limit his turnovers -- he is a smart player with high-end vision, which leads to hope in this respect. The fact that he is almost a full year younger than Power and Edvinsson lends hope to the fact that he has a higher learning curve defensively. But I would caution that the idea an offensive defenseman can be taught defense has largely proven to be myth. In fact, there are more NHL defensemen who did not produce at the amateur level who learned to be productive offensively than the other way around.
However, no one is asking Luke Hughes to be Jake Slavin. The hope is that Luke becomes an above-average defender to go along with being a 50+ or even 60+ point force from the back-end. I do think it is very realistic for Luke to grow into a passable defender who mostly takes care of business in his own zone with a few quirks -- he's not a complete horror show back there a la Poirier or Grans in the 2020 draft.