For the Germans to move an administrative role from Berlin to Brussels is little more than the move from Bonn to Berlin. the individual states in Germany will remain powerful and independent.
it's the same in Spain.
But France would have a problem giving up power to Brussels because it would go against 400 years of French history and admin
German politicians will need to become involved with every new law, no matter how small. The excuse, Oh, those people in Brussels did it, just wont fly anymore. And with this decision, European integration becomes part of Germany's domestic politics. Brussels is no longer just a side dish, but a main course; and local politicians are going to have to come to grips with what happens there.
As long as the constitution remains valid (and it can only be superseded by a national referendum), Karlsruhe is likely to have the last word on issues of jurisprudence and the constitutioneven over the European Court of Justice in Luxembourg. This could cause some tension and conflict. But anything is better than apathy.