There's an interesting article on PE Hub called "The Unicorn Dilemma" that discusses the lack of IPOs or M&A transactions involving unicorns this year. There's concern that these unicorns are raising late-stage venture rounds at valuations that won't be sustainable in the public markets. These late-stage rounds, sometimes called "Private IPOs" occur when the private company raises a significant amount of capital that can sustain their operations for several years, so they are under no pressure to go public. The PE Hub article discusses all of this and is a good article.
Link to PE Hub article: https://www.pehub.com/2015/07/the-unicorn-dilemma-vcj-cover-story/