Still not clear what the practical downside would be of a "united states of europe".
Scale mostly, increase size and complexity of any 'project' it becomes exponentially more difficult to manage (see Euro v national currencies for example). Diseconomy of scale, muddled compromises based on horse trading, unintended consequences increase and effects become less predictable, inadequacies of leadership and decision-making magnified, etc.
Reduction of diversity makes systems fragile as the scale of errors is magnified and 'firebreaks' are removed.
Reduction of diversity reduces the potential to find better solutions as fewer options are tried.
Lack of Europe wide public sphere means lack of democratic accountability with different countries living in significantly different political realities.
Centralising power incentivises lobbyists/special interest groups to influence legislation.
Nationalism will likely increase as countries perceive they are getting a raw deal while others are getting favourable treatment, or countries feel their values are being subordinated to the values of others.
Power will be moved much further from the people and political leaders will be increasingly detached from those they are supposed to represent and the consequences of their decisions.
Top down attempts to remake (large parts of) the world don't have a great track record of succeeding and more often make things worse.
Erosion of different cultures makes the world a duller place (and a successful US of E would require this)
I could go on...
What do you think the advantages are?