Avantages
Relatively flat hierarchy, with a friendly environment where everyone can casually chat with everyone else, even between the most junior employees and the CEO. A lot of very smart people around. The right kind of "agile" (that is, adapting goals to the evolution of the situation, not changing objectives non-stop because of poor prioritization). Stability: low turnover, with colleagues who have been in the company for a very long time. This translates to people that really understand their product and can establish long-term strategies. Good work-life balance (but this seemingly really depends on the position).
Inconvénients
The flat hierarchy can make some things inefficient or slow (sometimes it's good to have somebody in the lead to push decisions, or setting clearly defined accountability for projects). Slowly but surely turning into an actual big company, with the growing pains that come with this process. Compensation not quite market-competitive. Not much is planned for you at the company level in terms of career progression or training opportunities, it will really depend on you and your manager.