His first hedge fund ran for 20 years, and then his second one 8 years, during which he had very positive returns, but the strategy he used stopped working in 2002 (probably because everyone else jumped on it too).
You have the market itself which is like the environment, it can go hot or cold based on broad trends, and if that's all their was fundamentals would likely work well.
The problem is there the dumb prey you feed on may adapt and remove your niche you fit in. Even worse is there is no shortage of predators watching you and waiting for you to make a mistake and they'll eat you alive.