I joined a startup as an engineer about 30 years ago. I was one of the first employees and one of the last to get some 'founder stock'. I took a big risk (low initial salary and even lent them money to make payroll) but it paid off. When the company was purchased 8 years later I did fairly well. Not enough to retire wealthy, but well worth the risk I took. I am glad I did it, but it could have turned out to be a big mistake.

Everyone working for a startup might have a different story to tell. Some good. Some bad. Tread lightly if you wade into the startup waters. It CAN be very rewarding but it can also lead to nowhere.