Of course Stanford [1] paints it like a Disney movie without mentioning:

  * Share Class & Rights (e.g. common stock, voting, etc)
  * Tax issues and how they are structured (a friend had to pay a lot even before exercising due to bad legal paperwork)
  * Dilution
  * "Market price" nonsense for private companies
  * Other risks when exercising
  * Liquidity
  * Boom/bust cycles
  * Lots of growing changes might get them to "have to let you go" and you get nothing to show for

Basically, you should talk to an experienced lawyer before taking any offer like this.

[1] Stanford to continue legacy admissions and... https://news.ycombinator.com/item?id=44846130