Ventajas
Working for a UN agency, international exposure, plenty of room for innovation. Pay is decent. Benefits depend on your contract type. Pretty much zero-sum. ICAs get virtually no benefits while LICA and staff get excellent benefits. ICA base salary is higher than LICA salary, while staff (rare) get the highest pay and best benefits.
Desventajas
Petty competition among leadership is a big problem, directors have cult-like followings which creates a "clique-ish" sort of environment, it can be very toxic at HQ. High turn over is an issue due to job insecurity caused by policy. Hiring can be arbitrary. It's hard to get in as an entry-level professional unless you start off as an intern, which many people do in order to get a foot in the door... so there'll be 30+ year old professionals and 24 year old newly grads doing the same intern work. Lack of any career path definition.