Been working at startups on and off for over 20 years. In total I've spent more money on stock options than I've made from them.
I'm currently on my fourth because it's good experience and I needed a job after being laid off from a larger company. I'm a exec and have a decent amount of options and the company has customers and a path to profitability, but I'm still not counting on those options being worth anything.
Of my previous 3 startups. 2 were acquired and one is still chugging along and making a small profit with no exit plans. Total value of my stock was the aforementioned negative number.
Join startups for the experience, not for the paper money.
If they only give you 90 days to buy your options when you leave the company, don't bother working there. Its a scam.