Pros
If you want to build something, truly build something, this is a great place to be. You're afforded opportunities to impact the business in a meaningful way from day one. Opportunities to take on stretch projects to push the limits of your growth. Clear, open lines of communication to leadership across the organization. It's been the most rewarding and purposeful time in my career.
Cons
The spotlight is hot for those who seek it. It's a performance-first environment which leads to periods of uncertainty, naturally. There could be better communication from the top-down when it comes to personnel decisions to ease some of the uncertainty. Some might see swift changes in strategy/direction as a con (give a strategy time to grow) or a Pro (don't waste time in moving on from the wrong approach); I'll leave that up to the reader.