Yeah this is the big thing that keeps me from being 100% all in on Fantilli. There's just so many factors that might influence how Carlsson builds on where he's already at. Some guys like, let's just use an example in Lafreniere, or Griffin Reinhart, are already so advanced relative to their peers that they look really promising but when it comes time to adapt multiple elements of their game to higher level NHL play, they don't develop like other top picks.
Carlsson gets points in his favor because he's already adapted his tools and talent to play well against adults, and like you said, he has the trust of national team coaches for a pretty prominent hockey nation. But if the question comes down to is he going to be a solid 70 point two way 2C or a transcendent first liner who can tilt the game with his intelligence and decision making, you'd want to see more than most of us are able to; how adaptable he is long term and short term. If a deficiency in his game results in a f*** up, how quickly does he clean it up? What does he do to clean it up? I'm no pro but I imagine pro scouts need to look at these nuanced details closely beyond just highlighting the good and bad things a guy does in any given game.