Nuge actually had 32 points in his first 29 NHL games, and it looked like he was a bona-fide 1st line All Star type. Then, reality set in...
Hall had a much slower start with only 1 point in his first 7 games, and 16 in his first 30.
They were both physically capable of NHL play at 18-19, but I tend to agree with @HockeyGuy1964 that there was equally no need to put either in the NHL then. Okay, they ended up with respectable numbers on terrible teams that missed the playoffs, but their confidence and leadership would have been more developed with another year in training, I think.
In the history of the NHL, there have been about 12 players who really needed to be in the NHL when they were 18. Gretzky and Crosby come to mind. Unless you're at that level, it's not necessary, the more so for defencemen (and even more so for goalies).
There's utterly and completely no benefit to playing first overall calibre prospects in junior leagues for "confidence" reasons. Those players are almost always beyond that competition at 17 years old, are either impact players or borderline impact players at 18 in the NHL, and almost always turn out to be all-star level players in the NHL. I don't think there is a single franchise that would have sent Taylor Hall back.
The gap we almost always see between the top of the draft and the next tier is the gap between an NHL player and someone who soon be one.