Betting on Real Madrid was never been a wrong decision, but it can turns become wrong decision if they all in on Real Madrid and think Real Madrid will win 100%. Whatever the teams you bet and how safe the bet is, you should only bet what you can afford to lose, the easy way is only bet with normal amount that usually you gambled on sports.
Europa League was the example, I believe there will be someone all in to bet Leverkusen since they were unbeatable.
Betting on Madrid to win is not a bad decision and it will not even give high returns, but batting against them will give more rewards and it is likely to happen because Dortmund is also not a small team because they also have little Champions League experience even though Madrid has more experience and squad than them this season.
What happened to Leverkusen on the final day is a lesson to learn and it still happening to Manchester City too because they are losing to United despite performance in the League and the FA Cup.
Dortmund might be lucky to win the game, they have good players and Jordan Sancho is a good and a lucky scorer who can easily score for the team.