Many single factors can outperform in discrete periods however can face excessive drawdowns. Multi-factor approaches balance the cyclicality and volatility of individual factors.
Over time, selected factor strategies can have quite divergent performance, it's rare for one factor to give the best performance several years running. Investing in single factors in this context can potentially be quite risky.
Balancing trend-following factors like Momentum with contrarian factors like Value – these will typically exhibit negative long-term correlations – may result in more stable relative returns across a business cycle as compared to single factor strategies. Hence, the beauty of multi-factor approaches is the potential for smoother relative returns compared to traditional active strategies.