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.
Napoli and Barcelona this season do not look as good as they did last season, but as we remember, Barcelona in the last Champions League could not even get out of the group, this season they were more lucky with their opponents, but now they are weaker, so compare them with Arsenal is difficult. Porto remains a dark horse, but I would still classify them as a strong middle team rather than a top club.