Pros
Flexibility to pursue your idea.
Cons
Many, I'll keep them short
1) Poor remuneration - The CEO and COO are not willing to pay their employees the compensation they deserve. Believe me, I have seen top graduates get dismal amount of money because of top management's selfish attitude.
2) Lack of Professionalism - You'll be asked to do manual and physical labor in the name of start-up and lack of resources and at the end of the day you wouldn't even get a single word of appreciation.
3) Politics- Within 10 odd people there is politics and top management especially chooses to create this poor environment to keep their ears filled with what's going behind their backs
4) Hiring and Firing - I have had literally seen several people get fired in the span of xy months I spent there.