Were you a fan of the Habs when Max Pacioretty was in his first 2-3 years with the club? He was a similar animal—a big guy who was a good goal scorer but struggled playing against men in the NHL. He got rocked on many occasions and struggled more than Slaf has.
He asked the club to send him to the AHL and in his second season there it all clicked for him, he came back and looked like a veteran on the ice.
Slaf is learning on the job and his production is similar to Jack Hughes, a very highly touted prospect. He has size and talent, he just needs some more time to put it all together. I expect that he will be a 60-80 point player during his prime but as I said, not all drafts are created equal.
There's no crime in being wrong here but most of the time it's better not to take a hard stance on anything because things can change and it's hard to come back from, I speak from experience lol.