Mean reversion tends to work well in sideways, range-bound markets, but it can be dangerous in a strong trend. A stock in a genuine downtrend often keeps falling even after breaking the lower Bollinger Band — this is known as 'catching a falling knife.' Mechanically buying every lower-band touch can get you repeatedly caught at the start of a real trend, so mean-reversion buys should be paired with a trend filter and a firm stop-loss.