This mainly depends upon your political leaning more than it depends upon reality.
We can in fact use the European Union as a sort of practical example of the advantages and drawbacks of such a system.
Furthermore, I wish to tension. The most crucial omission from the question is that of the level of autonomy that each of the current countries, groups and factions would have under such a federal government. I’m presuming that we will have a high level of autonomy. Which we are just discussing the economics and politics of such a scenario.
If you wished to take this integration to medium autonomy, you could base this off of the structure of Imperial Britain. Or if you longed for there to be low autonomy and an extremely centralised federal government then you might base this off the United States federal system.
However, in the meantime, we will keep it at high autonomy.
Our world is already more linked than it ever has actually remained in all of human history. This would accelerate the pattern of interconnectedness and produce a fantastic boost in wealth across the world, particularly for bad nations. Poor countries would have increased access to the wealth of the industrialized world. While this would bring substantial investment opportunities for the middle and upper classes in the developed world; it would almost entirely mess up the working class in these nations.
We have actually seen this take place before. Especially in Britain, where increasing earnings and a more competitive establishing world destroyed the traditional market in the rural mining neighborhoods in Wales and the North of England.
As an outcome, anticipate brief, however more likely, medium-term political instability combined with an increase of anti-globalist belief.
Once once again, this can be seen in a lesser type today. Where after the crisis of 2008, many working individuals were hit hard and the middle class was severely harmed. The consequence of this is that the richest in society managed to get richer, while the bad saw their opportunities diminish. Many people would get annoyed at migrants coming and operating in their own nations.
Matters of migration are by nature, extremely questionable. While many argue that immigration is much better for society in regards to economics, and undoubtedly this holds true, the main concern comes at the cultural level.
Higher than sustainable levels of migration from greatly different cultures typically cause fragmentation within the wider society. Various cultures who do not necessarily get along will be pushed into close communities within cities. This is happening today. And will likely far go beyond assimilation level causing cultural pockets to reveal themselves in different levels of society.
This will likely be the largest concern dealing with a global government.
The problem being, that people want to cope with those they seem as being from the exact same tribe.
For an international government to prosper, there would need to be a body that controls the cultural makeup of different areas of the polity lest civil motions tear the union up from the within.
This is a difficult one.
Essentially, I personally would relate to an international currency as a catastrophe.
It would in impact, be a type of worldwide feudalism.
It would avoid poor countries from cheapening their currency to attract investment, keeping them poor.
I’m not exactly sure on the energy of this, as this has been recommended for the European Union – however it is possible to have a global currency with various levels.
I.e. a global currency for abundant nations, one for middle-income nations and so on
Regardless, the concern then relies on trade paths and human capital.
Do we keep things the very same? Where would the centres of administration be?
London, New York City, and others are excellent contenders. But then definitely a nation like China would feel left out.
This is the challenging part. When economics meets politics.
This is totally theoretical.
And then, while the world does appear like it is going towards a more globalised and integrated society, how does this come about?
A relatively loose union of all of the nations is possible. Anything comparable to the global federal governments from sci-fi books and movies is difficult to bring about without the use of force.