Those two situations are not comparable at all. SYRIZA wants a separate currency specifically so it will become weaker, allowing their country to be more competitive in the global market than if it was still on the Euro.
Cuba is a different situation. There was no flow of foreign currency in exchange for Cuban pesos, so they are worthless.
A closer analog to Greece are all of the south east asian countries that have their own currency, but it is weaker than the dollar & euro. Those seem to be working just fine.
First, SYRIZA doesn't want a separate currency, only the most radical members do. And their rationale has nothing to do with being competitive in the global market. Its widely described by their leadership as a dreaded last resort if its creditors continue to be unrelenting and force a default.
Cuba is a different situation. There was no flow of foreign currency in exchange for Cuban pesos, so they are worthless.
A closer analog to Greece are all of the south east asian countries that have their own currency, but it is weaker than the dollar & euro. Those seem to be working just fine.