Union Berlin formally now has the best defense in the Bundesliga, while Borussia's attack is far from ideal. Both teams are in the top group and it seems to me that they will play more cautiously than with the intention to get three points at all costs. I think betting on BTTS is very risky in this meeting.
Agree with what you said because indeed 9 matches have been run this season they are the club that has conceded the fewest goals because only 6 goals have been in Ronnow's goal.
But indeed this can still change because even in this case Union is still very good but I am a little doubtful whether they can still be consistent or not in the upcoming matches.
Now the main thing for them is to “endure” until the World Cup, after which, presumably, Union Berlin will have a more rested and assembled team relative to its competitors. As for the danger of betting BTTS in the upcoming game, an additional factor is Borussia itself, which can play an equal game with Bayern, and then fail in a "normal" match. Overall, I don't think the odds outweigh the risk.