I hope it's not that easy to say that Napoli will easily win this match. Even though it is true that Union Berlin is not as good as Napoli, Napoli itself is currently in a phase that is not very good. Moreover, they lost good players on their front line.
Union Berlin suffered 2 defeats in the Champions League, but that doesn't mean they can't recover to beat Napoli. In fact I also have Napoli as a favorite and I think most people would do the same. But they have to be careful, and also the luck factor might be something that makes this match different.
Union Berlin doesn't seem like a big obstacle for Napoli in the next match. Napoli, who previously lost to Real Madrid, could most likely win this away match without any problems, they also came with their entire best squad to the Berlin Olympic Stadium. Napoli are favorites in this match even though Union Berlin is playing in front of their fans, so there is little chance of it ending in a draw or both playing out a draw.
The Sevilla vs Arsenal match is also very interesting. Sevilla, who were held to a draw by Real Madrid in the La Liga match, may have to fight hard to beat Arsenal. Sevilla have only collected 2 points from 2 drawn matches, so they will definitely be motivated to win the match against Arsenal so that their chances in the group stage can be better.
Most chances Union Berlin will be having exit from this group after the end of group matches because their current form is also not good with things are looking not good against Napoli as well their first game against Real Madrid was impressive, but game against Braga was completely nightmare which give them shocking defeat and now having opponent like Napoli is also not easy most chances we will be having Real Madrid and Napoli from this group into knock out stage and Braga into Europa Cup.
Arsenal recent form is surely concern for them because they are having surprising defeat against Lens in France and now having another draw against Chelsea which is making things bad for them Sevilla is surely having good record at home for the European competitions with this most chances Arsenal will lose few points here as most chances we can watch draw or Sevilla win in this game.