I've worked at a couple startups as well as a couple big companies. My salary + bonus has always been much, much higher at large companies than the startups. I also worked much less, so my effective per hour pay was double or more. Perhaps I got the short end of the stick, but I've never seen a dime from my stock options at these companies. One of which was aquired for a decent amount more than the valuation of the company at the time my shares were issued. I suppose the preferrreds took it all. It wouldn't have been that much anyway. Maybe 50 or 100k.
In the best realistic case, you are looking at 3-5 years of engineering pay as a one time exit after years of putting in extra hours and probably being underpaid. That's been my experience at least as well as everyone I personally know who has worked in the Bay Area the past decade.
Certainly a lot of start-ups take advantage of engineers, I was more making the case that Box isn't really a start-up so I wouldn't expect them to have a big upside on IPO.
There are some specific reasons to join a start-up, and hitting the unicorn lottery shouldn't be one of them. Also, you should be taking a market salary and working sane hours, which seems to be more the case in the last 2-3 years, though YMMV.
In the best realistic case, you are looking at 3-5 years of engineering pay as a one time exit after years of putting in extra hours and probably being underpaid. That's been my experience at least as well as everyone I personally know who has worked in the Bay Area the past decade.