Union Berlin is doing well based on the investment they had and the money they invested there, I think the performance of Union Berlin was not bad with the players they have. In the last season, Union Berlin was even racing at the top of the table but compared to Bayern Munich and Dortmund they didn't have enough potential and lost their place I think they need much more for their teams to get better results during the winter.
Union Berlin is one of Bundesliga teams have good progress until success promoted to Bundesliga and last achievement finished on top fourth position, they don't have huge investor but their fans become part of this teams become success with fans support financial condition. I watched the video when Union Berlin fans took part in building the An der Alten Försterei stadium. But have very poor performance for the first time appearance in Champion League they failed defend top fourth standings in Bundesliga.
Depend with financial condition, I don't Union Berlin have much money for signing top players and actually some players sign this season have as free agent. Leonardo Bonucci can't help Union Berlin have good performance in Champion League and Bundesliga but still left chance in domestic league with many matches left for moving to top place. Union Berlin can sign some top level players when winter transfer window opening and try to finish on top fourth because they have many points gap from Dortmund.