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.