Well, hypothetically speaking, if we got a USE there would be several major issues.
-- For a United States of ANYTHING to be successful, it has to have a unified national civic identity, and it has to be widely accepted by all within those United States. This typically comes from some kind of foundation narrative, a war for unification, etc etc (EG, the Prussian Wars for Unification, the American War for Independence, etc etc). That, at this point in time, doesn't exist in Europe. For and Against European camps are roughly split down the middle, so any USE would already be on weak footing, since peoples national identities would still be "English, German, French, etc etc" rather than "European," which would make lots of infighting and destructive bickering more likely. Plus, you'd find that a lot of Germans would vote for the German candidate for presidency rather than the "best" one, because they would identify more with the German candidate. It wouldn't be like in the USA, where everybody regardless of state largely identifies as "AMERICAN." Considering that only 40% of Europeans trust the EU, and the majority of Europeans claim that the EU Council makes them feel "Powerless," such nationalism is a LONG ASS way off. So the USE would be disadvantaged before it even began.
-- The Euro. The shared currency of the Euro DEMONSTRABLY doesn't work; the Eurozone has been in crisis for decades now, and countries keep failing and faltering. Combining these economies into one would likely be disastrous and lead to several more recessions and bankruptcies, especially with a failing currency.
-- The sheer size of the population and variety of cultural identities NOT bound together through a national identity would create MASSIVE amounts of strain and pressure, to the point where a LARGE amount of peoples wishes are overridden by other peoples, and if a larger country votes for something and a smaller country (well, in this case, state) votes against something, the smaller state will be affected because it was basically outvoted by a larger member. Yes, this happens in other "United States," but with the populations being so drastically different (if the current EU became the USE, not including prospecting members, that would be 508 million people Vs the USA's 318 million, so the issues would be massively exacerbated) and the cultures being so focused rather than as spread out as they are in the USA, for example, it could lead to some countries feeling like they're being left out or ignored, which then strains the already weak European National Identity.
You might look at the USA and imagine a European version, but in order for America to work vast amounts of money are transferred from rich areas to poor ones and entire states are abandoned to economic decline because of the cost of saving them. It's true that large areas of the economy benefit from the US system, but others lose out as a consequence, which because of the concentrated national identities of the "States" in this scenario, many in a USE wouldn't accept since they'd feel EXTRA victimised since GOOD LUCK getting rid of the "English" or "French" national identity that has built up over thousands of years of history rather than the USA, whose states were NEVER independent nations. Not to mention, in the USA there was a civil war
to keep the union together and enforce the idea that the national government could override the wishes of the states. Can you even IMAGINE how brutal a European Civil War could be over that exact subject?
-- None of the states that stand to lose out from a USE would agree without massive concessions which would limit the power and authority of the central government, which in turn would weaken everyone else's perception of what such an institution would be like.
-- But assuming that you got past all that, and the USE became a Superpower Nation with "european Nationalism," would you even want it? Think about it. If you take that to its logical conclusion, you end up with a world made up of a handful of superpower federations, which will inevitably mean tensions as they chafe against one another. It might well end up being Europe, the US, China, the West and Central African confederation, the Islamic Republic, and so on. And historically speaking...having a group of Superpowers rather than dozens of minor powers NEVER ends well.
-- As for a United European army, police force, etc...well, that again requires Nationalistic European Identity, which doesn't really exist, and what if the UK or French sections get sent to a war the "state" doesn't want to be involved in? Again, this goes back to the whole "Used to be sovereign nations" thing, and is anybody under the impression that a unified European military WITH a European national identity would be a good thing? Look how much a nationalistic AMERICAN army has fucked up recently, with no oversight or people to answer to!
-- Do you really think that people in France, UK, Germany, etc etc would want to give ALL of their powers over to what they would consider a foreign council, so far removed from their own culture that they may as well be in a different continent altogether? The UK, for example, already chafes at the restrictions imposed on it by the EU courts and legislation, so how bad would the unrest get if we had to give over ALL of our sovereign powers? And that's assuming that the USE would be democratically elected; if it worked any similarly to how it works now, the president of the USE wouldn't even be voted for by the people, since the general populaces NOW don't have any real say in who's in charge over in Brussels.
-- You have the fact that everybody speaks a different language, there's NO cultural or historical common ground between - for example - people in the UK and in Bulgaria which would harm any unification attempts since people wouldn't identify with each other, different areas have different economies that won't mesh together particularly well, people are already a bit iffy about free movement between countries - how much worse would that get when ANY border control is taken out of their hands - AND the fact that if (when) the USE crashes due to its weak currency and the vast area it has to juggle, it would drag the world into another world recession since that would be a good...what, fifth? Quarter of the world suddenly experiencing a crash?
There's more, but I'm busy this afternoon so can't spend hours writing this. But basically, the USE would have MASSIVE pitfalls and obstacles to overcome, and even if they managed it, it would be a massive disaster from start to finish, especially with such a weak currency and filled with ALREADY failing countries that are dragging everybody elses economies down with them.
Anyway, I said I didn't really want a debate since I'm not the best with numbers and law - I'll freely admit that - and the possible USE is a long term concern rather than my immediate reasoning (my issues are more with sovereignty, the unelected nature of the EU Council and their habit of meddling in our internal affairs and to a lesser extent, the economy), so this will likely be my last post here. I have a lot of stuff to do on my day off, then I have a busy few days at work...and then it's after the Referendum, at which point this whole debate is kinda moot. So...thank you for the thinking points, thank you for correcting me on the Article 50...um, let's see....sorry if I came across as overly aggressive or condescending or anything in this - I tend to write off the top of my head, so I don't always phrase things correctly - and thank you for not getting angry at me.