Pros
Generally, people really do care about the mission and use it to guide difficult decisions. There's of course a spectrum, as with the employees of any company, but I found that it's less of a talking point and more of a priority at Affirm. Affirm was the best learning experience I've had by far. The people at Affirm are smart and motivated, and being surrounded by people like this force you to become a better version of yourself. There are good opportunities within Engineering to expand and try new things. There's an emphasis on personal development as long as you can align that with company goals. I'm generally quite bullish on Affirm and its success moving forward. There are definitely risks, but it's positioned itself well to succeed.
Cons
Sometimes aggressive business / product goals can be unrealistic, and it means that engineers end up working extra hard in order to get things done. Decisions and scoping is always done jointly between product and eng, but people generally expect that you'll stick to loose deadlines you've set for yourself, so it's important to scope appropriately. As with most tech companies, people sometimes get recognized for high visibility projects more than projects that are equally important but less well known. These are the main things that I can think of -- Overall, my time at Affirm was quite good; I ended up leaving because of personal reasons and not because of any issues with Affirm.