Euro - for other countries to use the Euro, they need to have Euro stocks. For this, the EU needs to (1) run a deficit, and (2) manufacture something of value to the rest of the world. But the EU won’t run a deficit, and its manufacturing sector is collapsing.
Yuan - China does not want rich foreigners speculating on their currency and causing financial crises, so they have limits on how much (digital) yuan foreigners can own. They do loan yuan to foreign governments, but those are usually given for specific projects like a dam or a railway line.
Both have issues:-