The first big issue is that not all of the nations of the world have a rich history of democratic understanding or due process or equal treatment under the law. Make a system too loose and you get the U.N. or the League of Nations, the former being largely ineffective at actually stopping war and the former outright failing. And if you don't institute a democratic structure in the countries, you run the risk of putting nations with terrible track records on human rights in charge of those things. There are some nations that do not think I deserve the same rights as other people and I'd rather not have them with that sort of power over my life.
The next issue is that if you allow for any government but control the power of voting by some metric of human development, than you'll find that the top 20 nations on the list Human-Development adjusted for inequity are all majority white and only one is Non-European (USA.)(Japan is the highest rated non-white majority country on the list at 21). Meanwhile, at the bottom end of the list, 18 are Black Majority (Haiti is the only one outside of Africa) and two are Middle Eastern countries (Afghanistan and Yemen). This is going to create a system where the most powerful countries are all white and the least powerful are non-white and for reasons that should be obvious to everyone, this will look bad.
Another problem is that Federations are prone to civil wars and crippling internal politics. Both the United States and Switzerland (big Federation systems users) had civil wars in their history. The USSR was a Federation of Soviet Nations and we can look to the early 90s to see how that played out. Malaysia finally had enough of the racial tensions in Singapore and just kicked them out. And those are some of the 27 extant federations in today's world. The list of defunct Federations is much greater and encompasses some nations that simply centralized or totally fell apart. With out very strict and well thought out safe guards, Federations tend to fail. In the United States at time of writing there are people who truly fear a second Civil War is coming soon and the two largest states by population have both threatened to secede in the past decade.
As to the "monopoly of violence" theory doesn't work. Even in Federation Nations, the sub-districts do have their own "power". The United States has the National Guard which functions as the state's military force. The Swiss Federation has mandatory Militia services for all adult males from age 20-30 (20-34 if you're an officer). The Federal Government does not hold monopolized power. The idea that this would save the world from Nuclear weapons is quite silly as almost all of the countries with truly large arsenals maintain "Second Strike Policies" which means they will only fire Nuclear Weapons if they are first fired upon by another Nuclear Nation. In fact, what kept the Cold War from turning hot was the fact that both nations wanted the other to attack first. Everyone was ready to fight a war that nobody wanted to start.
Where Federations work are in nations with a strong collective identity or morality... that is, nations that can agree on quite a few things. The ones that don't work are the ones that give too much room for differences to allow for unity of the whole.
