Comment by pas

there will be always inequalities and "blind spots", just look at the US, more homogeneous in many ways, yet still there's no single market for many things (healthcare for example)

education seems similarly harmonized in both unions (the Bologna system works pretty well)

but just as in the US border issues are always affecting members differently (migration flows North, right? so southern borders are affected more; at the same time migrants went to NYC and Berlin because they are rich cities with opportunities and very migration-friendly policies)

and of course federalism in the US is also suffering from vetocracy (aka. tragedy of the anticommons), see housing, which very directly leads to "blue states" losing seats in the House (and similarly housing issues are catalyzing radicalization in the EU too)

(and the solution to the housing challenges are not obvious, and even if there are success stories - like Vienna - city-state politics is stuck in the usual local minimas)