About the only thing the UN actually does well are it's humanitarian missions and that only works when their finances are under close supervision. If the UN is to endure as a global icon, it MUST make these missions the most important duty of their organization.
I think how well the UN performs in humanitarian missions (disaster relief and such, missions without any form of military or economic sanctions component) depends on several different factors.
One, the UN is required to operate with the consent and the co-operation of the host government. If the government says they'll handle it (whether or not they can or have the slightest intention to), there is nothing the UN can do. If the government says their own agencies will handle co-ordination and distribution of aid, with the UN just providing the money, it is anybody's guess as to what proportion of the money and relief supplies will actually reach those in need, and how much will simply disappear into the pockets of government officials. If the government allows the UN agencies to run the show, the UN will still usually be reliant on local partners to handle the aid distribution, as a situation where the UN will have the money and the manpower in place to set up their own networks in a timely fashion in some random cesspit of a country is possible pretty much only in
cause célèbre cases where there is a lot of money available from foreign governments and other donors. Usually the budget of an operation does not permit this.
Two, while there have been widely publicised cases of financial corruption within the UN, mostly the staff are normal people who do their jobs. The organisation is monstrously inefficient, though, with a top-heavy structure where many of the higher positions are held in sinecure by retired diplomats, politicians, and friends-of-cousins. Many times it is mostly the lower ranks who do what actually gets done, with the massive higher echelons keeping themselves busy with needless meetings, internal politicking, and navigating the vast morass of administrative procedures. It is somewhat like a national government in this respect, with the main difference being that all the regional centers are sovereign actors in their own right.
Three, unless they have lots of money at their disposal (which they usually don't), or they are working with a properly functioning host government which can provide accurate information and is willing and able to properly distribute aid (which they often aren't), a UN humanitarian aid mission tends to come too late to help in the crucial early days of a disaster. If trusting the host government amounts to idiocy, then the UN needs to gather information, assess the situation, develop a plan, perform a bidding contest for the procurement of aid supplies (relief supplies in UN regional warehouses are available faster, but are usually a lot less cost-effective than what can be procured locally, an important consideration for a cash-strapped operation), and set up a distribution network. By the time aid starts reaching those in need, several weeks have often gone by.
My view is that things like these, rather than the limited amount of cases where there is actual corruption within the UN, that make the UN terribly inefficient and sometimes ineffective as a humanitarian relief organisation. But it's what we have and the best thing we are likely to have for a good long while. In my opinion, if you retain the UN as a humanitarian relief organisation, you don't do it because the UN is the greatest thing since sliced bread. You do it because, without the UN, millions of people would likely suffer and die in places that don't feature in the evening news and where their own governments simply don't give a shit.
If alleviating human suffering is not a concern, then most of what the UN does on the humanitarian side amounts to a waste of taxpayers' money for any single first-world nation.
