Vanguard Group founder John Bogle has been a vocal critic of exchange traded funds in recent years. He fears individual investors could be tempted to trade ETFs too much and shoot themselves in the foot.

“There’s no question that ETFs are the greatest trading innovation of the 21st century,” Bogle said this week at the Bloomberg Portfolio Manager Mash-Up, Investment News reports. “But the question is, ‘Are they the greatest investment innovation?’ and the answer is ‘no.’”

He also thinks that with over 1,000 ETFs listed in the U.S., it’s difficult for investors to choose the correct fund.

The Vanguard founder said he has a bullish outlook on stocks and that the case for equities to beat bonds over the next decade is “pretty simple” but “not without a few bumps along the way,” according to the report.

Vanguard is the third-largest ETF manager with $187.3 billion in the category as of Jan. 31, according to ETF Industry Association data. Bogle is known for popularizing index funds in the retail market.

“His principles in terms of standing for investors have been formulated in a fashion of elegance and persuasiveness that few Americans have ever before displayed,” Roger G. Ibbotson, chairman and chief investment officer of Milford, Conn.-based Zebra Capital, said. [Vanguard’s Sauter Says More ETF Education Needed]