After the lockup expires, you'd better hope to your lucky stars that the earnings have been good, because those first few earnings are tough, and can sink the stock price. And then on top of that, there are probably very specific windows that you can sell your stock inside of, so you have to hope to time the ups-and-downs just right.
Lots of ways for that to bite you in the ass. I have friends who had this all line up the wrong way as Twitter employees who joined a year or so before they IPOed, and currently that stock isn't worth exercising.
http://www.reuters.com/article/2014/11/17/us-zynga-lawsuit-r...
STARTUP NEGOTIATIONS: HOW PREFERRED STOCK MAKES EMPLOYEE STOCK LESS VALUABLE http://stockoptioncounsel.com/blog/negotiating-equity-what-i...