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.
You're right regarding Barcelona's inconsistency this season. They've been inconsistent in LaLiga. Personally, I dont see them dominating the Champions League, especially against Napoli. Their domestic performance isnt great, and it raises worries about their Champions League fitness. Regarding Napoli, they may have the edge. They've played well and shouldnt be underestimated. Arsenal is powerful domestically but not invincible. Porto is a tough opponent, so I wouldnt be shocked if they win. Both matchups are uncertain, so it will be entertaining!