Real Sociedad are a much better team in the Champions League compared to the La Liga this season. They completed Group D as leader despite having a competitive opponent like Inter in the same group. Can they be a challenging opponent for PSG as well? I think that they absolutely can. You can predict the reason behind that easily too in fact.
It is true that Real Sociedad performed well in UCL. They could finish the group stage as the leader of the group D. It is quite interesting because they are in a better position than Inter Milan. We know that the point is the same, but Real Sociedad has lower conceded goals. It means the defense of Real Sociedad is quite solid, it won't be easy to defeat by other teams.
They will play against PSG in the next match, I'm also sure that Real Sociedad must have the chance to win. PSG seems not playing very impressively, they were almost eliminated in the group stage. PSG was also conceded many goals during the matches in the group stage. It makes me a bit doubtful with the performance of PSG. Their defense looks not strong enough to compete with a strong team. Although Real Sociedad attackers aren't top players, they could destroy PSG defense as it seems rather weak. Meanwhile PSG attackers will struggle a lot because Real Sociedad has a solid defense.
Real Sociedad was lucky to only meet Inter as the only strong team in the group stage, but if Sociedad were in group F then I am sure Sociedad would not be able to win the group stage or could even be eliminated from the Champions League.
Not all teams that are undefeated in the group stage and become group winners will succeed in qualifying for the quarter-finals if they have strong opponents in the round of 16. PSG is a strong team and will not be easy to beat, so it would be very unfair if you ignore PSG's chances without any consideration. Real Sociedad certainly has a chance to beat PSG, but PSG's defense is not as weak as you mean.
Real Sociedad luck is because they are in group D, you could say group D is one of the easiest compared to the other groups. In fact, Inter Milan deserved more to win the group, but they lost in terms of the number of goals and conceded, and Benfica and Salzburg were not comparable opponents, making it easier for Sociedad to advance to the round of 16. I predict Sociedad progress will be stopped in the round of 16 by PSG, the fact that Luis Enrique has often faced them will be a plus point, where Enrique also has better squad depth. On the other hand, even though PSG are not the favorites in the UCL, they seem to be able to beat Sociedad.
The reason that La Liga is more competitive than Ligue 1 makes sense, but in many ways Sociedad is still far behind PSG. It would be quite safe to bet on PSG winning, even though Sociedad is showing different performances between the UCL and La Liga. The priority scale of each club is very different, but balancing the teams in two elite European competitions is the main priority. Whatever it is, PSG is considered to still have a chance of continuing to advance to the quarter-finals, without ruling out the possibility that Sociedad will create a surprise because PSG is playing badly. But on paper, betting on Sociedad is very risky and should be considered carefully.
We know and really understand that PSG escaped the group phase because of luck, yes they were lucky to escape the hellish group. Their defeats to Newcastle and Milan have made us all doubt whether Enrique will be able to bring his fleet to speak much going forward. However, as I said at the start, Sociedad can trouble PSG, but they need a miracle to overthrow a team like PSG in the round of 16. Sociedad reputation in La Liga can be used as a reference, although not completely. I think their chances of advancing to the UCL quarter-finals are very low.