"Liquidity Risk
"“The dirty secret of passive index funds -- whether open-end, closed-end, or ETF -- is the distribution of daily dollar value traded among the securities within the indexes they mimic.
"“In the Russell 2000 Index, for instance, the vast majority of stocks are lower volume, lower value-traded stocks. Today I counted 1,049 stocks that traded less than $5 million in value during the day. That is over half, and almost half of those -- 456 stocks -- traded less than $1 million during the day. Yet through indexation and passive investing, hundreds of billions are linked to stocks like this. The S&P 500 is no different -- the index contains the world’s largest stocks, but still, 266 stocks -- over half -- traded under $150 million today. That sounds like a lot, but trillions of dollars in assets globally are indexed to these stocks. The theater keeps getting more crowded, but the exit door is the same as it always was. All this gets worse as you get into even less liquid equity and bond markets globally.”"
But other than that the market in index funds dwarfs that in the indexed securities, I haven't the vaguest clue what they (this isn't the first time I have heard this) are worried about.