I flew as a military pilot for a UN mission for 6 months, but I was also employed as the air liaison between our unit and the UN HQ to coordinate the taskings. Eye opening and not in a good way. On one side of the airfield we thought we were there to save the world (naive perhaps), the other side of the airfield was occupied by career bureaucrats who lived like kings with very little apparent concern for the country we were all there to help. Playing tour guide to the senior bureaucrats on their annual visits from NY didn't help in my cynicism. If I was to do it all again, I wouldn't.
If you want a sense of accomplishment perhaps a good NGO is the way to go, because I didn't get that from the UN.