Drawing on evidence from PRC province-level administrative divisions, European Union member states, and German L?nder, the paper highlights common challenges in decentralized systems in currency unions: opaque liabilities, uneven fiscal capacity, political constraints, and enforcement issues. The analysis shows that while fiscal rules can support discipline, they must also avoid complexity, procyclicality, and loss of legitimacy. The paper argues for context-specific, adaptable debt-sustainability frameworks aligned with growth, revenue capacity, and spending responsibilities rather than arbitrary numerical thresholds.