Yes you are right, that is why using matingale for strategy on gambling games is not good idea, it will need much big amount capital from gamblers and owner of website gambling.
Try to maintained not to reached the betting limit. Sports betting will do the chores. Around an average of 3 or 4 game with continous martingale bet for sure people have a higher chance to win since sports betting is not just pure luck.
This is my regular doing since I started sports betting and the output is good. Don't just add some risk by putting bet to other sports that we really didn't know. Also have a patience to find a good odds that worth to place a bet.
I think under that situation I would choose NBA or any basketball games that I know, its true that martingale really works and based on my experience it normally works in playoffs game especially in the Finals, because it is rare to see a team getting sweep.