Admittedly, but they all involve very strong internal transfers.
So the Euro could work if pensions, benefits, defense, border controls (yes, the Shengen agreement is a massive subsidy for Germany and other countries in the middle of it) were paid at the federal level. That would not be enough (cultural and linguistic differences would still mean that assymetric shocks would sting harder, so average inflation would have to be higher to help assuage them), but certainly necessary.

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi