Sports betting is a kind of hard to bet game sometimes, and I personally never consider the martingale system as a good betting strategy because I like to go for low-risk gambling.
You can consider choosing the team that always doing well in a certain league but you have to first consider doing some analysis on the probability of winning which their opponent have before making decision so you won't bet on the game blindly and later regret your decision.
Although I don't support the martingale system be it in sports betting or casino gambling but according to some people they said that they experience minimal losses when they use this system on sports betting compared to casino games. This might be true when you are able to be a little disciplined, although it's hard to maintain self control when using the martingale system. If your bet fails after four trials then it's best to quit
If I were to consider the legitimacy of this technique it has to come from someone who has done this a long time because it’s possible that they just got striked by luck. It also depends on the games they are playing and how much they are risking. I believe those factors affect the success of this technique in which we all know are different
from each otherper gambler.