I personally wouldn't list any cons, but I know that for a lot of people there are two things that might be a tough adjustment.
1. Hours. If you need a 7-3, 8-4, 9-5 schedule, then this is not for you. Hours are pretty standard but can vary depending on the situation (as most startup hours can). It is easy to get in a routine here, but your routine is likely going to be different from your friends and family.
2. You need to be thick-skinned. Let me be clear that there are never any sort of personal attacks and the leadership team does not play favorites. But there is honest feedback coming and going from all levels of the company, regarding ideas and work quality. A sensitive person would probably need to make an adjustment to feel at home here.