Sturgeon's Law says that 90% of everything is crud, and that applies to startups. The good ones are rare. If you get a chance to work at one of the good ones, you should go for it. But can you be sure? Sometimes the founders are talented at selling a false image of what the project will entail.
Startups are a chance to build something entirely original with brilliant and ambitious people. But startups are also dangerous. Limited money means there is little room for mistakes. One bad decision can mean bankruptcy. The potential payoff attracts capital, which in turn attracts scam artists. The unscrupulous often lack the skills needed to succeed, but sometimes they are smart enough to trick investors. Even entrepreneurs who start with a strong moral compass can find that the threat of failure unmoors their ethics from their ambition. Emotions matter. We might hope that those in leadership positions possess strength and resilience, but vanity and fragile egos have sabotaged many of the businesses that I’ve worked with. Defeat is always a possibility, and not everyone finds healthy ways to deal with the stress.
As an example of what can go wrong, see here: