Another of Mr. Gurley's concerns relates to the way entrepreneurs have delayed initial public offerings. By staying private for as long as possible, these companies often operate without rigorous financial and operational controls, which is to their detriment, he said.
Mr. Gurley's starkest warnings have been directed at the venture industry, now flush with more cash than at any time since the late 1990s. The flood of capital has allowed both good and bad companies to stay afloat, which Mr. Gurley said lets entrepreneurs engage in unsound business practices.
For example, start-ups are encouraged to compete with one another with price cuts and discounts when investors are willing to subsidize them. In the long run, however, price cuts are not a sustainable way to keep customers.
Thanks to all the easy money in the start-up system, "it could take years to know which business models really work," Mr. Gurley said. For investors, that could depress returns, he added.
Such opinions have sometimes made Mr. Gurley less than popular in Silicon Valley. Marc Andreessen, a founder of Netscape who is now a venture capitalist and who has been optimistic about the start-up boom, once called Mr. Gurley his Newman, a reference to the annoying postal worker in the sitcom "Seinfeld."
Mr. Gurley's bearish outlook has also opened him to teasing. At his 50th birthday party last spring, his family poked fun at his distaste for unicorn companies with a slide presentation showing the investor secretly tending to a herd of the mythical creatures.
Some said they appreciate Mr. Gurley's speaking out. Mr. Gurley is "calling B.S. on the relentless, blind confidence," said Roger McNamee, a Silicon Valley investor, adding that it was "brave."
That outspokenness has not meant Mr. Gurley is always right. In 2015, onstage at the South by Southwest festival, he said there would be "some dead unicorns this year." None died that year. Since the end of 2015, the number of unicorn firms has risen to about 186 from 144, according to data from CB Insights.
When the e-commerce start-up Jet.com garnered a $600 million valuation in 2015 before it had sold even a single item, Mr. Gurley tweeted his doubts and wrote "when I see this stuff I fear the end is near." Last year, Jet.com sold to Walmart for more than $3.3 billion, netting a big payday for investors. Mr. Gurley later said on Twitter that he had been wrong.
"He's respected because of how he behaves when he's wrong, and when he's right, he makes the point in a way that makes everyone laugh," Jon Sakoda, a partner at the venture firm New Enterprise Associates, said of Mr. Gurley. The firm was one of the backers of Jet.com.