Pros
They do great things, it is work that matters, it is exciting, well paid, many interesting and good people. You are part of the UN and there is a lot of pride in that.
Cons
For Staff: If you screw up they promote you out and away. Very many high-level people who should NOT be there (but they cannot be fired) so they continue to cause problems over and over again. For consultants: Staff get the credit/recognition, benefits, and job security - You do all the work and then have to apply for your job every 11 months. Projects are often under-resourced so consultants need to do the work or two or more people and they burn you out. I saw very many Country Directors and other high-level Staff who were clearly not up to the job, but somehow they are protected by someone and manage to get reassigned to another post and make the same huge mistakes over and over again. Office politics on steroids! The stated core value of integrity is rare in the senior staff and you would be wise to tread very carefully, document everything, take no verbal instructions, and watch your back! They will through you under the bus to cover up their mistakes.