I know it's been a while, but remember when he was praised for his performance in Game One of the TOR/BOS series in 2019?
He wasn't bad in his first couple of playoffs and then he had that strong showing. However, since then, it's been steadily downhill for him. This year was the worst showing for him so far in my opinion - visually he looked really timid and quite slow and ineffective physically; you really can't argue he "deserved" any better in terms of production or general on-ice results whereas maybe you can to some extent for other years.
I remember Nylander looked somewhat intimidated in his first couple of playoff series against the Bruins. However, since then he's noticeably grown as a player. Not only has he gotten physically stronger, he's shown willingness to step outside of his comfort zone at times.
They're both skilled wingers and neither will be mistaken for prime Forsberg, but one has demonstrated much more of a desire to get better and become more effectiveness in the playoffs than the other in the last five years. If anything, Marner has regressed in terms of compete and willingness to get his hands dirty.