I disagree with the generalization of this but I do agree its more often than not the case. You have to treat prospects on a case-by-case basis but there are trends to be aware of.
But Brannstrom is a special case because he did play a few times 20 minutes in the SHL, which is crazy and you rarely get in non-top 5 draft picks.
That said, I agree he should go back to the SHL and play another year. But I do not think there are ZERO good reasons from him to see him in the NHL this season. That are some, just some better and safer reasons to develop him slowly in the SHL.
Since 1990, I don't think a single player who defender who played in the NHL at 18 has lived up to expectations offensively. The best player is Brent Burns (who only played 36 games and was sent to the WJC and also spent time on the wing if I remember correctly). The other defender who spent a full season was Hamrlik who I don't think ever reached his anything resembling his offensive ceiling. The other guys were Bogosian, Kyle McLaren, Trevordovsky, Sbisa, and Aki Berg. The jury is still out on Ekblad and Hanafin, as they could be the exception. For whatever reason, the same doesn't appear to happen to 19-year-olds in their D+1 like Pronger, Doughty, Hedman, etc. But it is still a fairly mixed bag.
As for players playing their D+1 in the NHL on an expansion team, it seems the guys who waited a year to come out faired much better. It is basically Kariya, Yashin, Legwand, Klesla vs Gaborik, Hamrlik, R. Niedermayer, Stefan, and Falloon.
I'd say unless you are talking about guys who are physical beats that you should probably avoid taking the gamble. Yes, each case is individual, but it would be foolish to ignore history. Brannstrom may have played 20 minutes a game occasionally, but he still averaged the ice-time of a specialist in a league that allows 7 defenders to be played at one (12 minutes a game).