That's a good test to evaluate startups. How honest are the founders on their expected failure rate? If you're not a person they need to be lying to (i.e. not a potential/actual employee, investor, customer, journalist, etc), will they tell you that their realistic probability for success is very low?

If they can't do this, that's a red flag. Their irrationality will injure people in a relationship with them. They may disrespect others. Optimism is vital, but so is groundedness.