When the market churns flat I tend to outperform because I have a sizable yield component and my individual name alpha shows its strength. When the market crashed in 2008 I ended up the year on short positions so that really impacts longer run outperformance. In December I went on a shopping spree after being defensively positioned into it which left me much better off than if I had passively been long the whole time.
My point is that no one strategy fits all investing risk thresholds or environments. I like to aim for a nice 7-12%/yr with minimal drawdown volatility and hedges against nasty things happening. I look "stupid" if the market is up 20% in a given year and I am not. Over the longer term nasty things seem to happen every X years which has vindicated the approach thus far.
A flat market is in many ways the best place for actively managed long-short funds to differentiate themselves. Given where overall market valuations are we are probably past the point of double digit market gains for a while (although I could have said that years ago). So I think that skillful active managers will be better positioned. That said, I recognize that I am an active manager so 1. I may be talking my own biases but 2. I think I can differentiate real investment skill from charlatans, which is very hard for even very smart non-professional investors