Privately-held stock is riskier, because it's basically worthless until the acquirer has its own exit event. When you sell your company for stock, you're basically just trading stock in your company for stock in the acquirer. If the acquirer tanks before exiting, your shares are just as worthless as if you'd tanked before exiting.

Now, it can sometimes work out - I doubt Evan Williams is complaining about selling Blogger to pre-IPO Google for (presumably - terms were never disclosed) stock. But in other cases, I'm not sure the founders did so well - SixApart, for example, isn't quite the darling it was when it purchased LiveJournal in 2005.