I don't think Union Berlin played very well in the Europa League. That is, they didn't achieve any great results. Although, of course, this Bundesliga season they were lucky to stay in the Champions League zone. But I think that in the next Champions League season they will not have the opportunity to compete with other strong clubs. Maybe they will try their luck in the Europa League again.
Union Berlin success finish on top fourth standings position and make them qualify to Champion League for next season, but looking on their experienced in Europe League last season I don't sure with Union Berlin has capacity for talking more. Stuck on 16th round at Europe League after eliminating by Union Saint-Gilloise with aggregate 6-3 seems Union Berlin still not ready for playing in European competition. No doubt with Union Berlin has consistent performance in domestic league this season but can't make Union Berlin keep compete in Champion League next season actually if get difficult group stage teams when drawing.
Union Berlin has played very well in the Bundesliga. But their performance was not regular. Union Berlin's team is not very strong. Although Union Berlin has qualified for the Champions League, I don't think this team has the ability to perform well in the Champions League. Union Berlin will definitely need to add experienced players to their squad if they want to do well in the Champions League next season. If we don't strengthen the team, I don't expect a better performance from Union Berlin next season.