I think Nelson has a higher ceiling than Iwobi based on his potential for end product. If he can lean on that, theres no doubt in my mind he will be fine here.IMO, we've had a tendency at Arsenal to look at younger players in particular based almost solely on technical ability. If a player does not have the right personality (e.g. a bit of an 'edge') for top level football it doesn't matter about their "technical talent" they still will never become a top player (to play against a Barcelona in the CL) and the investment in that player has to be cut short. In some ways we've done this with Iwobi. Some would have liked to have kept him for another season but the club ruthlessly cashed in based on what they predicted was his ceiling.
I'm not convinced regarding Reiss, almost solely on personality, not technical, although I could be wrong, but I do sense he is on borrowed time right now. There is a clear personality difference from Saka, and it doesn't take a football connoisseur to instinctively pick this up even just casually watching the both of them.