Pros
The people are some of the brightest, most well-rounded human beings I've ever met. The problems being worked on are extremely interesting technically, and they'll have a huge impact on making energy clean and affordable. The product is fascinating to learn about, and there's really no shortage of ways to make it better. Despite being a small startup with lots of potential, Mainspring's culture is not toxic at all: there's no pressure to sacrifice your mental and physical wellbeing in order to crank out more work to one-up coworkers. In fact, the core cultural values are clearly stated and are designed to maximize longevity and general satisfaction at work, while moving the company's goals forward. Employees seem happy and have families/a multitude of hobbies outside work. The leadership is excellent, and super down-to-earth. There is a lot of trust built between the difference layers of the organization. Everything is very transparent, and I can tell we are on a positive trajectory.
Cons
There are no major drawbacks that I can tell. Of course, every startup is going to have a bit more work (on occasion) than the usual 9-5, more architecture churn than established companies as requirements/priorities change, and a bit less "predicability" in roles/responsibilities. However, Mainspring manages it well. It feels like there is just enough support to make things work, and teams are continuing to grow.