They are too many to name, unfortunately. Some big ones to highlight are:
1. Dishonest and Untrustworthy Senior Management. They are inexperienced, unethical and unprofessional -- never in my career has it been so bad. Instead of addressing problems with the product, pricing, sales, they instead shift blame on employees. In effect, a toxic culture of distrust, dissatisfaction, and ultimately round after round of lay-offs occur. For example, they fired a Co-Founder out of nowhere. You will likely follow a similar fate.
2. Low Salary. Be sure to negotiate. The equity portion is nearly worthless - don't believe that you will have any real stake in the start-up. If you are in Sales, the quota is unrealistic. No one really hits it.
3. Product is Unreliable. So client renewals/client churn is a big challenge because the startup cannot deliver the service at the high price point they are charging.
4. Unprofitable. And that creates an environment of anxiety and poor accountability. Every single employee and every team is raw on edge.