Vantaggi
The WHO houses some of the best public health technical expertise in the world. The opportunities to learn from colleagues' professional experiences and technical know-how are plentiful. The public lectures/talks available to staff are high-caliber and it's truly an "international" workplace which is great. Geneva is also a beautiful, relaxing place to live (albeit grossly expensive).
Svantaggi
Too much emphasis on technical research capacity and enough bureaucratic red tape to suffocate oneself. The actual "doing" in the field gets done by contracted private companies or NGOs; the HQ in Geneva is more placed to provide technical expertise and serve as a hub for political engagement. I was often frustrated with the laissez-faire attitude of some staff who were clearly only there for the UN "perks" and were far less concerned with having an impact in developing countries. The approval process for projects was long and grueling. In the end, I chose to leave WHO because it was far too removed from the real, tangible experience that comes from working in the field.