Two forecasters who got it right:
There are others, but two will suffice. From: http://www.bloomberg.com/news/articles/2015-07-15/nine-people-who-saw-the-greek-crisis-coming-years-before-everyone-else-did
First there was Wynne Godley, a British economist wrote about his own concerns in a 1992 article for the London Review of Books:
What happens if a whole country—a potential ‘region’ in a fully integrated community—suffers a structural setback? So long as it is a sovereign state, it can devalue its currency. It can then trade successfully at full employment provided its people accept the necessary cut in their real incomes. With an economic and monetary union, this recourse is obviously barred, and its prospect is grave indeed unless federal budgeting arrangements are made which fulfil a redistributive role. ... If a country or region has no power to devalue, and if it is not the beneficiary of a system of fiscal equalisation, then there is nothing to stop it suffering a process of cumulative and terminal decline leading, in the end, to emigration as the only alternative to poverty or starvation.
And second, Arnulf Baring, a German political scientist (?) who offered dire predictions in his 1997 book Scheitert Deutschland? Here's an English translation:
They will say that we are subsidizing scroungers, lounging in cafés on the Mediterranean beaches. Monetary union, in the end, will result in a gigantic blackmailing operation. When we Germans demand monetary discipline, other countries will blame their financial woes on that same discipline, and by extension, on us. More, they will perceive us as a kind of economic policeman. We risk once again becoming the most hated in Europe.