Vantaggi
You will work for the United Nations for the betterment of humanity's health and wellbeing, along with so many incredible, intelligent, motivated and wonderful people. It's a real honor, and the colleagues at your side are what will get you through the day (in many cases). Also great pay (tax free if on a staff contract) and good benefits.
Svantaggi
The system of management is completely broken, at all levels. Technical staff are promoted into management roles with no management skills at all. Archaic systems are made more unbearable by a hierarchical system separating technical staff from 'support' staff, even in extremely taxing administrative positions with huge responsibility without which the Organization would not operate. Focus on holding meetings and producing reports, without any consideration of how their work, time and precious money should be spent most effectively. Work life balance is terrible unless you flat out refuse and insist on leaving - if you show any inclination to work hard it will be taken advantage of to burnout and beyond.