I think there's no need to discuss about Barcelona vs Napoli and Arsenal vs FC Porto, it's a sure win for both Barcelona and Arsenal, Napoli and FC Porto aren't that good compared to last season.
Barcelona have also been inconsistent this season, especially in the LaLiga, but mind you that every match in the CL RO16 is going to be tough, i understand that some teams are underdogs when you compare them to their oppositions, but in the two games you highlighted above, neither of those teans are underdogs, it is going to be a 50-50 game that anybody can win. Barcelona and Arsenal are very much beatable, and it would be a mistake to write off Napoli and Porto.
Well, in the round of 16, matches often end in surprises. We have to remember last season's round of 16. At that time, the favorite team stopped in the round of 16. And maybe this time the same thing will happen. But for the Arsenal vs Porto match, maybe I will still have confidence that Arsenal will be the winner and qualify for the next stage. But for the Barcelona vs Napoli match, I think this one is more difficult for us to predict. Even Barcelona is not as good as we think at the moment. Lewandowski even seems to have lost his sharpness this season. This 35 year old man even had difficulty scoring goals this season. So I also prefer to think that the Barcelona vs Napoli match has a 50:50 chance. Because anything can still happen. Napoli also seems to be trying to focus more on Serie A and the Champions League because they have been eliminated from the Coppa Italia.