I recently went to the Swedish Army Museum, and there was a really nice exhibit on Swedish involvement in the United Nations Operation in the Congo (ONUC) during the 1960s. There were several interviews with veterans involved in the operation, and in most of them, the veterans were critical of how the UN handled the situation as well as the strict engagement rules. Was this simply a soldier's frustration at not being able to fire back? Have these missions been successful in their primary objective of keeping the peace from a broad 20th century view?
I will only be able to talk about post 1991 UN actions, because that's what I know the most about UN effectivness.
After the Cold War, the world became more unstable than before. The fall of the Soviet Union allowed a massive amount of weapons to be diffused in unstable areas. Most of these unstables countries will fall into civil wars, ethnic wars, religious wars, all violating Human Rights. The international community will soon be advocating for the UN to send peacekeeping missions there.
In 1992, in Yugoslavia, the UN sent 38 000 soldiers to protect civilans and create safe areas from the ethnic cleansing organised by Serbians and Bosnians during the Yugoslavian war. This conflict is really tricky, so I'll make an exemple for you to understand it better. Disclaimer : what I will say applies to both side, Serbians and Bosnians, this is just an exemple :
There is a Serbian population in a village inside a Bosnian majority area. Bosnians want to either kill the Serbians or force them to leave. This is called ethnic cleansing. What can the UN do ? Move the Serbians to a safer area ? This would mean playing the Bosnians' game, because you will force the Serbians to leave their homes. So you defend the Serbians ? Reminder, in a UN peacekeeping mandate you cannot take a side neither shoot if your life is not threatened. So you can actually do nothing, because the Bosnians will aim for the Serbians and not UN soldiers. And that's what happened during this war. For this reason, UN peacekeeping force wasn't able to stop the Srebrenica massacre, and were recognize guilty for having left alone to their fate civilans.
However in 1999, UN will control with quite good result the Kosovo region that will gain its independence in 2008. If the UN cannot prevent conflicts, they managed to help repairing the consequences of conflicts as in Liberia in 1997 or with the creation of an International Court for ex-Yugoslavia in 1993. The same court was created in Rwanda and Liberia before becoming permanent in 2002.
So what can we say about the UN actions post 1991 ? Well, the result is far from perfect, for one particular reason in my opinion : the way we wage war has changed. Most of actual wars are not interstates wars, but civil wars. And in civil wars, civils are aimed at. And that makes the UN mission difficult, because they cant choose a side to defend, they can only try to protect the most that they can and it's always not enough. It took time and blood for the UN to understand what the best approach to these kind of situations. The UN learnt their lessons from Yugoslavia and today manage better their interventions.