With the creation of a clutch of new financial watchdog agencies, the European Union this week took its biggest step so far to prevent a recurrence of economic crises that have thrown the future of the euro into question. But the overhaul will not eliminate bickering between countries over how to run their economies, and it could take time for some of the more far-reaching powers — like the imposition of temporary bans on harmful financial products and directing recommendations to national banking supervisors — to gain acceptance.