Based on everything. He's not good defensively, not physical, has no shot and doesn't put up points on a stacked offensive team and his hockey IQ is questionable.
He's one of the best players in the league at zone exits, rushes the puck as good as anyone, can absolutely dominate play in the offensive zone at times (limited opportunity to do so), and has an elite breakout pass with elite skating. His conditioning is top notch (broke the twist record w/ a 60" vertical) and has been since his junior days. Logging 25 minutes a night won't be/isn't an issue for him. Also, no shot? His wrist shot is very good, it's his slap shot that is non-existent.
His low PP time is the cause for him stagnating at around 30pts/year, and is a product of Babcock wanting him to focus solely on his defensive game at this early stage in his career, rather than him not being capable (as per his words).
He has every tool to become a good #1D.
Risto has some of the worst underlying defensive numbers of any defenseman in the league. He likely has a slight edge on Rielly at this point in their career but that in itself is arguable, while the difference between the Nylander's is massive, both today and moving forward.