Pros
You're not only designing a launch vehicle from scratch, which is already awesome. You're also shaping the culture and values of the organization as it grows with incredible speed. You also get to develop an entirely new manufacturing platform that will revolutionize the way large aerospace structures are made. All of this cool stuff aside, the reason this company will be successful is because of Tim Ellis and Jordan Noone. They are the reason the culture is awesome and sustainable. They are the reason company leadership is able to put egos aside and focus on achieving goals. They are the reason Relativity is a better place to work than other space startups. They care about their employees, and it really shows.
Cons
Hypergrowth. It's hard to stay ahead of it, and sometimes the leadership falls short on communication. This is only temporary though, and voicing concerns along the way will be met with pragmatic solutions from management, not mere head nods. Benefits (aside from stock options) are average, but they will improve as the company grows and is better positioned to negotiate with providers.