Betting on Real Madrid is certainly not a bad decision, there are many advantages of Real Madrid compared to Dortmund which makes bettors have no doubts about betting on Real Madrid. All predictions point to Real Madrid winning, this belief arises based on Real Madrid's ability and experience in the UCL.
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.