In summary, we have a monetary union without a common fiscal, tax, social and defence policy that has failed and continues to fail: Greece's bankruptcy, Brexit, Catalonia, high unemployment in Italy, Spain and Slovakia, to name but a few disasters.
SEE CHART
https://www.zerohedge.com/news/2018-01-18/euro-disaster-failed-monetary-unions-past-present
My problem with the Euro remains the same since it was rolled out: a single nation state currency (ie US dollar, UK pound) is backed by the full faith and credit of said State. The pride of the nation backs those currencies. The Euro is backed by nothing other than bureaucrats and marks on a ledger and if the going ever became really bad the member nation states would pull out to save themselves.
Almost happened with the Greek crisis.