Pros
Takes care of employees and offers very flexible work environment / schedule. There's also an opportunity to get $10K a year in education reimbursement (for a degree, not a certificate program). Good pay and job security.
Cons
There's no real growth trajectory. There are very few opportunities to experience challenging, meaningful work on a consistent basis. Also, most of your peers will have been at Fannie Mae around 7+ years and performance management is heavily weighted towards those with more tenure - not necessarily more demonstrated talent and skills. While "innovation" is encouraged there's very little new product development or autonomy to experiment with new ideas. Most people are happy with the status quo - but a talent refresh is certainly needed.