Napoli and Barcelona may not be in their best form, but Napoli are no more experienced in the Champions League than Barcelona. I believe Xavi can get his team to play better from now on and win more games, but he has to be very careful against Napoli. After all, Napoli still have great players who can give Barcelona suffering until they fail to qualify for the quarter-finals.
Currently both of them are not in their best performance, we can say that both Barcelona and Napoli are experiencing a little difficulty in optimizing their performance. However, fortunately, Barcelona has a coach who is actually quite solid at Barcelona, while Napoli is still in its early days with Mazzarri. So maybe Barcelona should have a better chance. But, Napoli also have great players, so this will be interesting..
Experience sometimes affects their performances in the Champions League, because with experience their mentality will be more tested. Last season Napoli performed very well, but in the Champions League they performed mediocre and even they were eliminated by a team that also came from Serie A. Their mentality has not been created very well, because the atmosphere in the Champions League is very different from the domestic league, even though their opponents also come from the same league.
But considering the performances of both are still not able to show good consistency, so it is difficult to predict this match.
Of course PSG must evaluate well so that they can advance to the next round because the match will be more difficult than we imagined in the round of 16. Real Sociedad cannot be underestimated, they are one of the teams with strong ambitions to become champions this season, even these two teams have never met before, so it will be quite difficult to predict the final result. Even though PSG has quality players in their team, if they can't make the most of the first leg then they will have difficulties in the second leg.
Real Sociedad is clearly still too far away from becoming champions, maybe their targets in the Champions League are a little different, at least Real Sociedad may want to show their existence first to the strongest teams. Because this is UCL where there are still candidates such as Bayern Munich, Man City and Madrid, of course Real Sociedad is aware that their current strength still cannot exceed that. So for now Real Sociedad will try to risk everything against PSG, so if they are able to beat PSG until Real Sociedad finishes in the top 4 then they can become a promising candidate. But it all requires support that is not small, conquering the UCL is not just about ambition but about the mentality of the players that a club has.
Compared to other teams including their opponents this time, Paris Saint Germain, it is clear that Real Sociedad is still below Paris Saint Germain or the other teams you mentioned here. But I agree with you that their target may not be a title, but reaching the semi-finals is already a great achievement for Real Sociedad, so there is no reason Real Sociedad can be underestimated in the Champions League.
Real Sociedad are one of four teams from La Liga to make it to the last 16, and if we look at those four teams, Real Sociedad are the weakest. But again I say that doesn't mean with such status they will be easy to beat.