Last year was a banner year for IPOs because the equity indices' run of 2013 carried over into the next year's investor sentiment, but with the flattening in stocks this year—the S&P 500 is up 3 percent—the opportunity has narrowed.
Even the best bets in the IPO market are beginning to cool down. Led by biotechs, the health-care sector stayed active, accounting for half of all IPOs in the first quarter, but that action is slowing. "The wind is out of the biotech sails right now," Gaskins said.
Several biotech IPOs scheduled to price last week haven't yet made it to market, and even those that did have disappointed in pricing and early trading. "I might have said that you could go to market anytime with biotech, until about a week ago," Renaissance Capital's Smith said.
A U.S. and world economy that are not growing significantly, along with lots of market uncertainty, shortens the time horizon investors afford to start-ups to break even.
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"The IPO market is healthy for profitable companies that have growth prospects," Gaskins said.
"There's a risk in companies that are very fast growing and don't have earnings and are spending a lot of marketing dollars," Smith said. "Anything can happen along the way, because we're in an environment with lots of capital sloshing around on the private side, and when a model assumes a start-up will be a dominant player but others are being funded, the onus is on the tech companies."
Or, in other words, many start-ups with good stories and the backing of Silicon Valley still have a lot to prove.
The 2015 CNBC Disruptor List will be unveiled on Tuesday. Follow coverage of the companies that made this year's list on CNBC and CNBC.com.