From Michael Harris of Price Action Lab in a Forbes interview:
There are some chart pattern traders who claim that although most patterns fail if they can be right 20% of the time they can profit because they make on the average a lot more than they lose, ie, the payoff ratio is high. The first problem with this claim is that a low win rate exposes the trader to a large risk of ruin. Even when tossing a fair coin a long streak of tails is possible before the relative frequency starts converging to 0.5. Now imagine what can happen if the coin is biased 80% tails: a long sequence of losing trades is more than certain.