1) This is not a stable job. Most startups fail. By joining a startup, you are buying into a high-risk/high-reward scenario.
2) If your offer to go work for them is not high-reward, odds are it is a bad offer. Be sure your compensation in the case of success has the potential to change your life. If it doesn't, go find a more stable job.
3) Most startups are led by people with limited experience, advised by investors with their own agendas. Their leaders tend to be young, ambitious, and smart. But not experienced. Sometimes this is terrific, and they do things in new ways and kick ass. Other times they are utter fools who drive the company into the ground.
4) VCs fund startups to make money. They do not care about you. They barely care about the company. They care about whether or not they are getting money. This forces startups to make high risk choices pushing for fast growth, not safe choices pushing for a stable company.
Overall, just be sure you know what you are getting into. Startups can be amazing experiences, with tons of learning, and good compensation. They also can be toxic pushes to burnout and bankruptcy. They are gambles. If you know this and go in with open eyes, then enjoy the ride.