The match between Barcelona and Napoli is the only UCL playoff match with 50-50 chance. Although I believe Barcelona has an edge.
Both Napoli and Barcelona are struggling at this moment. But this is the UEFA Champions League and it brings the best out of everyone. Barcelona has a legacy and a history. Barcelona won their last match against Almeria but they struggled really hard. I don't see Barcelona finishing above the 3rd place this season in La Liga unless Girona messes up really badly. Atletico Madrid are above them now.
Napoli on the other hand just suffered a huge loss in Copa Italia against a mid-tier team. They are in 5th place and not looking very good. Both Barcelona and Napoli will be hoping to get back before their matches in February.
Barcelona and Napoli are currently struggling, I think Napoli is way worse. But In the Champions League, teams often find a different level of motivation and performance, and past struggles in domestic leagues might not necessarily carry over to European competitions. Barcelona have legacy and rich in the Champions League, with multiple titles and memorable performances that suggests an ability to rise to the occasion in crucial matches.
Champions League brings out the best in everyone is a crucial point. I think Barcelona is more familiar with this tournament that give them advantage in mentaity aspect. Both Barcelona and Napoli will be hoping to recover before their matches in February. This is the importance of the period leading up to the fixture, where teams can address their issues, regain form, and approach the Champions League clash with renewed confidence.
I understand your concerns regarding Barcelona's Champions League chances. They've done well, but Napoli has too. Yes, they're struggling in Serie A, but European competitions typically show otherwise. Napoli has shown some significant skill in European contests recently. Barcelona may struggle with that. The January transfer window is wildcard. Both teams could add players, changing the dynamic. Barcelona may face a Premier League team later, but I'm not writing them off. Their high-pressure game experience may help. Again, the Champions League's volatility makes it intriguing, right?