Avantages
A lot of freedom for developing stuff and using new technologies. Minimal bureaucracy, informal internal interactions. Interesting product to work on with variety of real-world use cases. Open communication with the management. International team, no nationality bias.
Inconvénients
In fact, the same as pros. Lack of structure lays an extra baggage of planning your activities and counting your progress. Minimum of processes in the company sometimes makes it hard to understand how to move forward a career inside the company. Lack of monetary encouragement for the work well done. Technical problems sometimes are really hard to crack.