Pros
Incredibly mission-driven, and therefore, you get to work with like-minded. Because they maintain a really high talent bar, you are also working with incredibly talented and smart people who have high cross-cultural competence. At the stage of growth when I joined, they were still under 5 years old, so you feel like you are part of building what will become in the future -- I literally helped to build infrastructure and processes. I had a lot of access and face-time with senior leadership too. As with any job, when you work with a good manager, everything seems better -- that was my experience here too. I had a great team and great managers.
Cons
As with any start-up, scaling and identity crises creep in. There were constant changes, and uncertainty. There was a time when that sort of thing was exciting, but it tires with time as one ages and craves more stability. This is especially hard when you are moving for your work, but have incomplete information about visas, housing etc. Finally, as it's still a relatively small and unknown organization, it's not the hugest building block for career (or savings for that matter). It takes a special person to take on this work -- if you are that kind of person, you will have enough job satisfaction.