Although the shift to ETFs over the past ten years has been dramatic—since the start of 2008, the industry has attracted nearly $2.8 trillion in investment dollars (according to the Investment Company Institute)—Vanguard chief executive Bill McNabb told The Financial Times that he doesn’t see this as a “systemic” trajectory indicative of a market bubble.

In the interview, McNabb argues that index tracking funds represent less than 15 percent of the equity market capitalization around the world, and less that 5 percent of daily trading volumes in global financial markets. [Vanguard, the article says, has “set industry records for new business inflows for five consecutive years and is growing at a faster rate than arch-rival BlackRock.”]

According to the article, however, last month Howard Marks of Oaktree Capital questioned whether ETFs and index mutual funds would be able to find buyers for their holdings if the market tightens, quoting him as saying: “When the management of assets is on autopilot, as it is with ETFs, then investment trends can go to great excess.”

McNabb says, however, that it is the more esoteric, complex ETFs that could be cause for concern. “I am more worried about ETFs that you don’t know what they mean,” he said, suggesting leverage as a component that investors “don’t understand the negatives [of].”