The other side of the coin, however, is that "there will always be a liquidity problem for the markets any time the amount of sellers swamps the amount of buyers," said Halftime Report trader and CEO of Ritholtz Wealth Management Josh Brown. "ETFs are way more efficient and although there will always be hiccups, the machinery has demonstrated its ability to be extremely durable in times of market stress," Brown said. The Bank of America note points out that, like it or not, ETFs in the U.S. may still have a long way to go. The bank says almost 70 percent of all assets under management in Japan are passively managed ETFs — more than double the percent of assets in the U.S.

http://www.cnbc.com/2017/07/05/etfs-may-lead-to-a-market-liquidity-problem-bank-of-america-says.html