I don't know about the opinions of groups, but personally I see the UN as a toothless organization that has political power and very little physical power. When push comes to shove they are useless. There have been instances in which they were on tbe ground near a massacre and could do nothing about it. 

Historically they have allowed Israel to get away with war crimes because of US voting in the UN. I can't recall any specifics, but there have been numerous votes where everyone except Israel and the US voted on the other side of a resolution. 

Ultimately I am unsure that any global organization can maintain legitimacy over the long run.