Pros
This is a fast-paced startup in a fantastic market space. That helps to provide a very positive vibe as people know they are changing the IT security landscape. You work long hours, but management appreciates everybody's efforts and provides lots of tokens of appreciation (snacks, bringing in dinner, recognition). This is not the first time at the security startup rodeo for most people, and it is great working with and learning from security veterans who know their s--t.
Cons
An early stage startup will evolve and perhaps pivot as it figures out what customer wants. It can be a bit of a roller coaster with some long hours, but that is part of the A-round startup experience.