To be fair, he's 18 and was being leaned on in a men's league and on a national team. If he's smart and coachable (which anecdotal evidence suggests he's a humble self aware kid) you have to think he can clean up anything technique wise. If he was too slow to keep up or too small to use his stick or position himself correctly I'd say it's a big ask but what you mention sounds more like just learning to pick your spots. We saw Jack having to go through that when he arrived. I'm sure Nemec will have to as well. He oozed skill and smarts at the prospect scrimmage against kids who for all intents and purposes were all older than him. That's a good start. I don't see anyway NJ doesn't pour coaching time and effort into him. If he doesn't improve where he needs to it will be a surprise unless he's not willing to put in the effort which seems unlikely to me.Pre-draft I was super, super, super high on Nemec. Still am.
But I think this is kind of the wrong read on him. His upside is, no question, a high-end offensive defenseman. But I don't think he's bust-proof. He's a bit of a riverboat gambler - he always activates to create chances. But that creates a lot of rushes against. And Nemec himself is a very poor transition defender. He's prone to the occasional bonehead turnover/gaffe.
If he hits, I think you have a Morgan Rielly. But he'll need to improve his spot-picking and transition defense. Otherwise, North American coaches might not trust him enough to play him.