When they started their careers, I thought that Marner was a try-hard on the defensive side of play while Nylander was usually well positioned defensively. Nylander definitely didn't use his body or try hard enough at times, but he is a smart player on offense and defense.
What always stuck out to me was how Nylander would nearly always get the puck when there was a board battle and he was the 3rd man in because of his positioning while other players battled for the puck. As long as he keeps up his effort defensively, he's definitely not a liability.