It's not unsubstantiated. Unless the only thing of substance to you is the ranking of the majority of scouts.
If you look at Kakko's skill set and his ability in tight spaces and corners, you know how well his game should translate to NHL. On the other hand, Hughes needs to be at a very special level for him to be such a good player in NHL, simply because he cannot bring the physicality or the strength. Even if his skating is as good as McDavid's, he loses to McDavid in every other aspect. And McDavid stills struggles on the cycle, he generates off the rush.
McDavid's pretty effective at cycling with the puck. He's very smart along the boards and does a good job of forechecking.
Hughes doesn't have that in his game. Hughes' game is very rush-centric, and in-zone movement only occurs on the powerplay. McDavid also generates chances off the rush, but he does so far better than Hughes does, and he has the strength to overpower the opposition when he's at top speed. Hughes isn't as shifty as McDavid.
Some compare Jack to Patrick Kane, but Kane is effortless on his edges -- not the fastest straight-line player, but incredibly shifty even while knifing through center at a casual glide. Hughes is more speed-based than agility-based. Kane also has a blistering shot, while Jack does not.
Hughes already struggles to find space at even strength at the U20 level. His boardwork is not good whatsoever -- puck retention is poor as a result of his lack of strength on his skates --, and he lacks an NHL-level shot. Unlike Elias Pettersson, who is actually
extremely shifty on his skates and handles the puck as well as anyone in the NHL, Jack actually can't deke players from a standstill -- he generates offense with speed. Hughes is a small, rush-centric, pure playmaker. He's a good complementary piece, but I think 60-point potential a la Keller and Ehlers is more likely than star potential.