Basically Barcelona still has a chance to win against Napoli in the last 16 but ya, even though Barcelona has a chance to win but I personally cannot consider the results they got in the group phase . Because anyway, I think the opponents Barcelona faced in the group phase were also easy because ya, at least Barcelona still has better quality than Shakhtar and Porto as you mentioned.
This will be one of the most challenging competitions in the 16th century. Barcelona vs Napoli. Both of them have not been at their best this season due to several problems occurring within their clubs. But at least, they are strong enough for each other, so they still have a chance. It's quite difficult to predict, but it's possible that Barcelona will come out on top if you look at how prepared the club is so far. Moreover, Napoli is having basic problems, especially those related to their coaching,
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.