What is the measurement for success? If we're talking stats, who cares? It's going to be a new/different system, with different scenarios on a play-by-play, game-to-game basis. He could throw for 3000+/- yards and have the most yards at QB in the division.
The only metric that really matters is wins, I guess. Even that is subjectve because again, new/different team. The schedule he's playing isn't even the same. Darnold had to win 12+ games in that division last year. Something tells me he won't need to in the NFCW. AZ is nonexistant, SC is a shell, and the Rams suck (biased opinion on this one). Seattle could take the division with 9/10 wins.
So again, what is the actual basis of success or failure he's to be judge on? This is why I despise the offseason, I see no value in conjecture, lol. Outside of friendly banter of course.