The UN is little more than a gathering of countries, it has no army of its own, and it really has no power except the power of the member countries. To blame the UN is pointless as it is not the answer to everything but it does give legitimacy to any action by one country to another by collective agreement of the community of countries it represents.
The UN can only highlight problems by bringing them to the world stage, without the UN there would be no forum for world issues, there would be no condemnation of militaristic countries abusing weaker nations. The UN can only approve or condemn what a country does it can’t make countries act, it is up to the individual countries to act.
An example would be East Timor, The UN and it’s agencies helped it vote for independence, the UN approved countries going in to settle the unrest after the vote, the UN helped set up a new government, and now the UN has approved countries entering the country again to settle it down. The UN couldn’t force Indonesia to allow a independence vote, the UN couldn’t enforce law and order in the country after the vote, the UN could only help form a government but couldn’t stop it from making mistakes leading to another peace-keeping force required… Only member countries can send troops and police for peace-keeping, only East Timor can form a decent government the UN can’t force one.
If you see injustice in the world don’t think the UN will fix it, lobby your government to do something, help a charity to do something, get on a plane and go help… Blaming the UN is pointless.
The UN is little more than an internationally governed charity with a large debating chamber for the world.