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.
I concur with your appraisal of the CL RO16 matches between Barcelona and Arsenal. Barcelona's poor LaLiga season may affect their Champions League performance. Barcelona's LaLiga struggles may affect their match against Napoli. After difficult seasons, Barcelona just returned to the last 16. Barcelona is a changing team striving to find its identity, making them susceptible.
However, Arsenal's scenario is intriguing. Their return may provide complications after a long absence. It will be difficult to beat Champions League-winning Porto. Arsenal has never won this title, and Porto's expertise in high-stakes contests may prove crucial. I agree - both matches are open-ended, and underestimating Napoli and Porto may be disastrous.