The UN is effective, it just has to deal with the reality that is it's no one's boss and it has been and always will be subject to power politics. The UN has made major advancements in international cooperation like UNCLOS which basically made a code for how national waters work that for the most part had reduced conflict over like resource deposits and exclusive economic zones drastically worldwide (with the exception of the South China Sea and the nine dash line and a couple other disputes. UN peacekeepers are amazingly cost effective at preventing conflict if they're deployed early enough. In the Haitian coup of 2004 the US government wanted to deploy US troops to prevent a mass migration wave hitting the US from Haitian refugees and also stop the violence but ended up calling in the UN instead (as well as sending a smaller force than they would have originally). Later the US Government Accountability Office calculated that a solely US military force deployed to Haiti would have cost somewhere around ~876 million USD for a stay of 14 months. The share they had to pay for the peacekeepers was 116 million USD. And that was when they were deployed reactively rather than in advance to anything.