Laine has shown in the past he can be an elite scorer, which arguably is the ultimate objective. But he’s not shown to be an elite player. He can’t drive his line, he can’t create turnovers. He doesn’t have elite speed, he doesn’t win board battles, etc. Scheifele did all that (albeit not lately).
He most certainly didn't show that consistently until the last half of his third year. I think some people forget how much of a project he was back then, he couldn't stay on his feet, got consistently muscled off pucks. It wasn't until his third year when he was 22 that he really started to show the ability to physically dominate other players and really drive a first line.
I don't think you will find many people who will disagree with the notion that Laine doesn't have areas to improve in his game but his weaknesses in a lot of ways are the same as Marks at the same age. I have no doubt by the time he is 22 and near his physical prime that he will be much stronger on the puck and will be able to drive his own line.