I've worked at startups that have had exits and the employees got zilch. I think even in the startups with exits the probability of an employee having their equity turn into cash money is very low (also I doubt 50% of startups have exits - I hear a number of 10% more often and even that seems high, I'd imagine 10% of YC startups have an exit and they are the most likely to succeed so I imagine when you add in all the other startups that percentage gets far far lower).