In startup culture you don't always know where the guardrails are, and not every task or role is clearly defined. Often, if there is a problem it will be imperative on you to not only define it but to push the solution. Depending on your mindset, these are not drawbacks, especially if you take a creative approach to solving problems of all types: technical, communications, policy and cultural included. That said, people with long experience in more hierarchical structures, those who expect responsibilities with clearly defined parameters, may find this way of doing business too free-form.