The UN is responsible for many organizations that help plenty of nations. It is also highly responsible for keeping the peace in many countries. Where the UN becomes limited is when countries are not willing to participate in something that is for the greater good. For example, the negotiations on emissions of green house gasses. America which was a huge country in regards to these emissions was not willing to participate in the negotiations to cut these emissions because they did not think it benefited them economically.
When you have a group like this it can truly only be effective if the large power countries are willing to participate. The only thing that could be done is to force countries to participate but that seems very unlikeily and maybe even impossible to accomplish. The UN is also very limited in issues regarding people being mistreated in their home countries. They often have to stay out of these humanitarian issues. I think The UN should be able to get involved with these issues if another nations agree that the problem is big enough.