The UN is as good or as bad as the commitment to it of its constituent countries. If you're going to have rogue nations (e.g. Russia, North Korea, USA, Israel) ignoring the will of its members and doing what they want then it's going to be ineffective.

The IMF is very good at what it does. It's not designed to help problem economys out, it's designed to enforce western financial hegemony over poor/developing/problem/strategically valuable nations.