Pros
Been at MongoDB for 6 years. Every time I grew restless the company was willing and helpful in making me grow my career. I have been consistently encouraged to take risks, move into different roles, and when that didn't work I could go back without taking a hit in my reputation. MongoDB is very good at attracting and rewarding strong performers, and giving them room or proactively pushing them to grow. Also, when experiencing personally difficult times my managers were always very supportive.
Kontras
As a fast-growing company sometimes it scales faster than the systems in place; it can feel that supporting systems (HR, IT, payroll...) are sometimes struggling to catch up with the growth. Work can be stressful at times, but not more than other places.