Pros
Surrounded by extremely intelligent people who are genuinely nice to be around, working on interesting and (for the most part) impactful work. Collegial atmosphere with people always willing to help each other to learn and grow, particularly certain senior experienced staff. Huge opportunities to advance very quickly for the right people and if you put the work in. In addition, opportunities to take on a great deal of responsibility very quickly. Work arrangements can be very flexible if you gain trust.
Cons
Sometimes developing country work can be slow and difficult to have an impact - this can be frustrating but is the nature of some of the work and not the firm. With a growing firm you don't get the same brand recognition outside the specific field as other career opportunities . Managers will not shy away from providing tough feedback in ways that some would find difficult to handle - but as others have said, if you can look past it and develop a thick skin (which is useful generally) you will learn a tremendous amount