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.
Barcelona performance is not good this season, even though I don’t know what they will play in the round 16 as the competition is going to be tough in this stage. However, the Barcelona and Napoli matches will be somehow unpredictable to me because both of them are not in good form, and we don’t know which team will dominate the game or the team that will be lucky to win that day. Although I think Napoli performance is quite better than Barcelona performance in the competition, you can see that the round 16 is going to be tough, so I think Napoli will do great.
As for Arsenal and Porto, I don’t really know, but I think Arsenal is doing a little bit well, so let's see how they perform when the day arrives, but I don’t think Porto will be able to win Arsenal.