There's no doubt that Union Berlin performed well this season especially during the start of the season and I can remember that they have been at the top of the table and have contested in several ways but it is unfortunate they were not the Champions and it is fortunate that they will appear in the Champions League.
The reason why they fluctuated in their performance was lack of squad depth. I believe they will fix the problem this season by signing more good players and players will likely reciprocate because they are now in European competition.
I agree with this, they need more solid players in the squad, the players they have this season have not been enough for them to maintain consistency so they have lost their place. They even lost their place before the first half of the season ended, because at that time I predicted they would maintain the top of the standings at least until the first half of the season, but in fact they didn't.
I do not think that it would be wise to expect them to easily get great players. It's not easy to find great players within their budget, sometimes you can and sometimes it's a fail, when you are a huge club you could have issues like Eden Hazard type of transfers or Coutinho etc, but most of the time you are getting an established star, so it is not really a problem for you at all.
But, when we are talking about a lower level team, you can afford to spend maybe 2 or 3 million euro at best on a single player, and that's the best case, most of the time you can't even afford to spend that much. This is why it's quite important to find the gem in the rough and sometimes like this season they do, but sometimes those players leave for better clubs and you have hard time replacing them.