Nearly every trading strategy is one of two things. It either bets that a move will continue, or it bets that a move will reverse. Trend following is the first; mean reversion is the second. Understanding which a strategy is, and what each needs from the market, explains most of what happens to it.
A trend follower enters after a move has begun and holds while it continues. It is wrong often, because most moves do not become trends, and it makes its money on the few that do, which it rides for a long time. Its win rate is low, its winners are large, and its losing periods are ranges, where it buys the top and sells the bottom repeatedly. Its strength is that it needs no forecast; it waits for the market to move and follows.
A mean-reversion system enters after a move has gone unusually far and bets on a return toward normal. It is right often, because most stretches do relax, and it makes small gains frequently. Its losers are large, because the stretch that does not relax is the start of a trend, and its losing periods are trends. Its strength is that ranges are common.
Neither is better. Each has an edge in the market state the other fears, and each hands its gains back in the state it was not built for. That is why the two are natural partners in a portfolio, and why a strategy that is only one of them has a record with long stretches that look like failure and are simply the wrong weather.
When assessing any strategy, ask which it is. Then look at its record in trending years and ranging years separately. A trend follower that made money in a range, or a mean-reversion system that made money in a trend, has either found something unusual or been fitted to the period, and the second is more common.