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.
They scored at least one goal in their last six games and three of them ended up with 2+ goals for them. But only two of them were wins though. Maybe Union Berlin are the least conceding team with only 6 goals. But Dortmund have a good record against them recently. They defeated Union Berlin here by 3-0 before. But this time it feels like this will be a really close game. Union Berlin are good at scoring also now.
Although it might seem risky, I think both teams to score is still worth it. Union Berlin might score first and Dortmund would score the equalizer. It is kinda draw game to me.
Yes, I understand your reasoning and the statistics seem to show that BTTS is a more likely outcome than not. The problem is in the odds - in order to make an average profit, you have to make a profitable bet. If you could bet BTTS with odds of 1.9, it would most likely be profitable. And 1.71 is a fair odds (without your edge) - you may or may not win, but I don't think that if you take 100 of these games and make 100 bets, you will end up with a profit.