Ventajas
The company culture is incredible. Open, interdisciplinary collaboration means that even as an intern I was able to voice concerns and have them taken seriously by experienced engineers. They treat interns nearly like full-time employees. While there is an understanding that you are learning, you are given serious responsibility and independence, and you are expected to contribute value. Knowing that your work has an impact on the final product is a big source of motivation. The talent that Rivian attracts is top-notch. Entry-level engineers have amazing abilities, drive, and talent, and older engineers or management all have experience at high levels at other companies. Things change very fast, which is exciting - progress can be seen every day.
Desventajas
Things change very fast, which means that goals may change and the scope of projects can increase; it can be hard to keep up. Roles and responsibilities may not be clearly defined, which can lead to some chaos. On the whole, this is beneficial to a dynamic startup, but can be a bit difficult for an intern to get in the flow of things. Make sure to clarify questions as you have them!