This is especially true in this time where even successful companies drag their feet when it comes to IPO. If you don't think you can't afford to buy the options and pay the taxes (yes, there's taxes when exercising illiquid options), then you have to discount the chances of still being with the company at the time of a liquidity event.
This means you can find yourself valuing stock options very little, even in cases where you have full faith in the company making it and being very profitable. A growing company is very different year to year. What are the chances that you'll love the same company in 6 years?
My personal calculation is that I have a 50% chance of leaving a job every year, so if I am guessing that it'll take 6 years to IPO, and I don't think I can just keep large amounts of money frozen in options, I have to discount the value of said options over 95% on top of the traditional calculations. RSUs from he big four don't have that problem.