You can experiment on this approach. I don't see anything too special or too risky with this betting method. The level of risk depends on how much money your starting bet is considering that you intend to do a Martingale. Just make sure you still have enough bankroll in the unlikely event that you lose for 4 consecutive matches. If you start with $20 on Barcelona, for example, you have to prepare $620 for a Martingale up to 5 in a row.
I ran the Barcelona simulator and it was a bad result because I did the math as you said and not on the odds, so after seeing the actual odds of the game here is how it looked based on the example 6 posts above with Bayern, I needed to take look at the odds also.
- match against Real Sociedad at 1.9 I need a 22 euro bet! Loss
- match against Celta at 1.7, I need a 60 euros bet! Loss
- match against Las Palmas at 1.15 I need a 680 euro bet. Loss
- match against Mallorca at 1.6 I need a 1200 euro bet. I win!
I lost only 3 times I needed 2000 euros for 20, that's a 1% return over 1 month, it's close to staking rewards, so not working for me!
You can also bet on a draw, I guess, with high odds. Start with pretty low bets, increase it by 2x every time your bet is lost, and I'm sure if you are lucky, the reward should be massive.
2x doesn't work, that was my mistake also, sometimes you need 10x because the next match has 1.10 odds.