Warning signs are emerging for Slack Technologies Inc., the business communications software company valued at $17 billion that goes public this week. Some Slack employees have been selling their stakes in the start-up ahead of the listing, according to data from secondary marketplace Forge, per Business Insider. This rush to sell shares is happening as some analysts express concern about the company's rich valuation amid slowing revenues, which could rein in the cloud company's share growth when it trades publicly. These bearish forecasts follow disappointing debuts from other tech giants like Lyft Inc. (LYFT) and Uber Technologies Inc. (UBER).

Like fast-growing Uber and Lyft, Slack has a major weakness. It's still losing money and is expected to post a $187 million adjusted operating loss at midpoint despite a 47% to 49% rise in sales in the current fiscal year, per Barron's.

Silicon Valley Employees Cash Out

Slack’s upcoming direct public listing on the New York Stock Exchange scheduled for June 20, similar to that of Spotify Technology SA (SPOT), differs from a traditional IPO in the sense that there will be no capital raised or issuance of new shares to investors. Nonetheless, the rocky public debuts Uber and Lyft have spooked many employees at Slack and other Silicon Valley companies, which traditionally have reaped major returns when their startup went public. But with the surging amount of capital in private markets and the trend for companies to stay private longer, employees in the startup world are worried that an IPO will actually devalue their holdings.

What It Means For Slack Investors

Forge, the marketplace that allows early employees at unicorns to sell their stakes to large money managers and hedge funds, says it saw a 100% increase in employees offloading shares at some companies in the month prior to their startup's expected IPO, per BI. Forge's VP of marketplace, Javier Avalos, told BI that employees at Slack are seeking to cash out as much as half of their stake in the cloud-based communication company before the IPO."The easiest thing for us to do would be to take some chips off the table" and give them liquidity, said Avalos.

The lackluster debuts of some unicorns are leading many employees to believe that they may get a better return on the secondary market. Slack shares on the secondary market have jumped as it gears up for the direct public listing date -- by as much as 250% from its series H funding round about one year ago, per BI.

Looking Ahead

Despite some employees' and analysts' caution about Slack’s direct listing, bulls say the stock will thrive as it disrupts the enterprise software industry. Slack counts more than 10 million daily active users across 600,000 organizations in 150 countries through its workforce collaboration software, per Barron’s. Now, Slack is moving beyond its core service and aims to disrupt email, which is seen as "an ineffective medium for sharing and managing information.”