Liverpool each struggle with their front lines, Nunez still hasn't contributed anything to Liverpool so it's very difficult for them to be included in potential champion candidates, while Napoli is an inconsistent team, we know in the last few seasons they have played normally but in this season they are looking good, if Napoli meets a big team like Madrid, PSG or Bayern, Napoli will surely lose.
Nunez hasn't been all that bad, he has scored a couple of goals and assisted some too, and if you judge his contribution with the minutes he has been restricted too, you'd understand that he has been somewhat impressive, though when i watch him i see that he still has a lot of work to do in order to become a better player, his decision making sometimes is quite poor. Liverpool aren't strong favorites because the overall team isn't playing at their best and not because of any player in particular.
Having said that, Napoli will be that smaller team that the big teams will be hoping to avoid, not that they cannot defeat them, but because it could be tricky, and absolutely anything can happen, i get this feeling that Napoli are going to provide an upset in this season's CL, not that they'd win it, of course they can't, but they might just eliminate a team that nobody gave them a chance against.