Pros
MongoDB is the most mature company I've worked at, in a number of ways. First, I find the people I work with easy to get along with. People are interested in doing excellent work, but it's a low-drama environment, which I appreciate. Second, the company itself has a mature vibe. I really appreciate our CEO's demeanor and focus at our All Hands calls. In general, it seems like the executive team is focused on long-term success, not short-term wins, which is great. I also find the work I'm doing pretty interesting, and there's a lot of potential to learn. The work-life balance is good, and the compensation is excellent.
Cons
We're a mature company. I think the big downside here is that we're big enough that we're not always super agile. In particular, when we need things from other teams, it can be hard to get things on their schedule (no doubt they say the same of my team). I think this comes from being a large company that is trying to be fairly organized and plan ahead. That means that every team has a long roadmap, so getting our thing onto that roadmap can be challenging. This doesn't mean that folks on other teams don't want to help. They're super helpful! But when it comes to larger changes, their hands are tied by the planning process.