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, I think you have a valid opinion on this and I think that Napoli and Porto have the same chance of overthrowing the superior team that is their opponent. Barcelona has had quite a hard time lately in the domestic league and they are still struggling to get back into the competition for the domestic league title. Usually the results obtained in the domestic league will also reflect their performance in the Champions League competition and that is why currently Barcelona may not be able to completely become the leading team against Napoli. In fact, this is the first time Barcelona has reached the last 16 after two seasons ago they dropped to the European league after failing to qualify in the group stage.
Arsenal may be very strong in the domestic league but this is also the first time they have qualified for the round of 16 after a long hiatus. Of course, they will also adapt and will not necessarily be assured of the victory that has been assigned to them. Porto is a tough opponent to beat, especially since they won the 2004 edition of the Champions League and Arsenal have never won the Champions League title. So yes, everything is equal in every way and this will be an exciting match.