Most latest sanctions by General Assembly or Security Council in favor of US. In case for war in Iraq and Afghanistan UN did nothing. The US Army and Marines killed thousands of civilians and none was brought to justice (almost none, except few idiots, who was not able to cover up their crimes). George W. Bush, Condoleezza Rice, and Donald Rumsfeld should be in prison now.
They cannot define one and true religion from various religions in the world that people can follow for success in second and eternal life hereafter which is very important for every person. They could not finish poverty and they are unable to establish peace in world forever. In reality, the reason of war among countries, poverty and bomb blasts is differences of religions and it is impossible for United Nations to stop war, establish peace and end poverty without finding and establishment of one true religion of the God.
U.N is driven by politics, not humanitarian concerns. Sometimes I think the U.N have good intentions with their "let's provide aid for everyone and keep the peace" motif, but those little peacekeeping mission honestly don't work, unfortunately. I really wish just going up to a dictator and telling them "hey, listen man, no one really likes you and you've lost all support of your people, why don't you just give it up?" would work, but it doesn't--they're power-hungry maniacs cowering behind their armies, they'll only give up by force. U.N has some good ideas but they don't have the balls to do it.
They can't get anything done because you have China and Russia (two communist power houses) blocking any good headway that the US proposes. Honestly, we should kick them out of the UN or just demote them. To this very moment, they haven't come up with a good solution to disarm Syria's chemical weapons.