I came across a great article explaining the pitfalls of martingale strategies and how they can be alluring but with time they all fail – catastrophically! - not sure who the author is but you will enjoy it nonetheless.
The martingale strategy asymptotically implies infinite capital or infinitely divisible stake. In reality you have a limited capital, the lower limit and in a casino also an upper limit of staking.
In a fair game (with 50/50 chance of profit and loss) the probability of profit after a series of losses is still 50% (because the outcomes of bets or trades are independent from each other).
Typically, if you win then your profits are moderate but if you lose, the losses are severe (you can lose your capital just after a small series of unlucky bets).
If you make many bets, you might make a good profit in the short term but the probability to make profit at all consistently decreases with the number of bets and the length of time. Losses always remain severe.