The major drawback of the United Nations is that it does not work as well as some would hope it would. This can be a bad thing because people can put too much faith in the UN and fail to do things that will make up for its deficiencies.
For example, if a country expects the UN to be able to keep the peace, it is fooling itself. The UN has not been able to prevent wars from happening in any real way. If countries expect it to prevent wars, they may not prepare to prevent wars themselves. If this happens, they will be unprepared to prevent wars and wars will become more prevalent.