From what I've seen of Isak I don't think he's as creative as Nunez, we don't disagree there. He definitely doesn't work as hard, but his touch is pretty immaculate. Either way i's probable that in a more positive team, those xA numbers would improve. I like Gordon, and I like Barnes when he's fit, but otherwise he's not had much support around him.
My gripe would be that if Isak shot more often, those xG numbers go up, and you rate him more even if he starts underperforming against that xG. Seems simplistic to say the least.
He's not as creative no, in fact his rate of xA is pretty stable from Sociedad to Newcastle, likewise with Nunez from Benfica to Liverpool.
His touch is immaculate yes and that's valuable. He plays the same amount of passes per game as Nunez (22) but his pass completion is 78% v Nunez's 70%, but that makes intuitive sense when you consider Darwin is trying more difficult passes and creating more whereas whenever I've watched Newcastle Isak is very much just there to lay it off into wide areas - which isn't a criticism.
The key bit here is 'if Isak shot more often'. The thing is he doesn't. Why? Could be a few things - level of service, him prioritising more high value shots or him being unable to generate more shots than he currently is due to a technical/physical limitation - i.e. doesn't have the acceleration to create separation v PL level defenders. Again, I don't know for sure.
It's the age old thing of people saying they'd score 10 goals a season if they played up front for Man City. In general I think people underrate how important/difficult it is to get your own shots. For example put Danny Ings number 9 with Foden and Grealish either side and KDB in behind he doesn't suddenly become a 4 shot per game striker.