Unicorns suddenly looking a lot less enchanting

Unicorns beware, more pain to come: O'Leary

The unicorn club is filling up, and it's causing unease among investors.

The number of privately held tech start-ups valued at $1 billion or more now tops 140. But there's a widespread concern on Wall Street that because this new breed of start-ups isn't profitable and can't live up to the super-high valuations, companies are staying private a lot longer.

This year has seen the lowest number of tech companies come public since 2009. According to Renaissance Capital, there have been just 22 tech IPOs in 2015 thus far, compared to 55 in 2014.

When Fidelity and BlackRock marked down their investments in companies like Dropbox and Snapchat in November, alarm bells were rung. Has the bubble burst?

Read MoreThese start-ups are still raising tons of money

According to PrivCo, a company that provides financial data on major privately held companies, these tech names are reluctant to go public because of the shaky IPO market. The companies that have gone public end up facing little flourish, and have found their worth on Wall Street is often lower than in the private market.