Catalonia represents 14% of the population and pays 22% of the taxes. The tax rate is both unfair, and harmful, since it is the one region of spain that might form an innovative industrial heartland, in southern Europe. Furthermore, Catalonias speak a slightly different language and have a distinct culture and have tried repeatedly to gain independence from spain, only succeeding for a brief period.
Federalism only works if a culture is homogenous. That is the problem for the US, and for Europe,