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.
The Napoli we saw last season can comfortably beat the current Barcelona team, but it's quite unfortunate that the Napoli we are seeing now are just a Shadow of the last season team. This match is going to be keenly contested. Both Barcelona and Napoli are not doing very well in their respective leagues. Both Napoli and Barcelona can advance to the next round so it will be difficult to determine who will qualify.