ISO 20022 messaging is a widely adopted standard for financial message formats. While it offers many advantages, it also has some potential flaws and challenges. Beyond the early claims that ISO 20022 messages couldn’t be put into APIs – which has been largely solved and demonstrated over at ISO20022 Plus Here are a few notable issues that continue to challenge ISO 20022 implementations globally:

1. Complexity: ISO 20022 is a comprehensive and highly flexible data and messaging standard. However, this flexibility can lead to increased complexity. Implementing and understanding the standard can be challenging, especially for organizations with limited resources or expertise. Customizing and adapting the standard to specific business needs may require significant effort.
2. Migration challenges: Many financial institutions have existing proprietary messaging formats and systems that are not compatible with ISO 20022. Migrating from legacy formats to ISO 20022 can be complex and time-consuming. It requires coordination between multiple stakeholders, including banks, financial market infrastructures, and software vendors. The migration process can involve data mapping, system upgrades, and testing, leading to potential disruptions and costs.
3. Cost implications: Implementing ISO 20022 can involve significant costs for financial institutions. Apart from the expenses associated with migration and system upgrades, there are ongoing maintenance and support costs. Small or medium-sized organizations can find it financially burdensome to adopt the standard.
4. Variations in implementation: ISO 20022 provides a common framework, but there can be variations in how it is implemented across different regions, markets, and financial institutions. These variations can make it challenging to achieve seamless interoperability and standardization. It may require additional effort to ensure consistent interpretation and usage of ISO 20022 messages.
5. Learning curve: ISO 20022 introduces new concepts, message types, and data fields, requiring users to become familiar with the standard. Financial institutions and their staff may need training and education to understand and effectively utilize ISO 20022 messages. The learning curve can add to the overall implementation time and costs.
6. Limited backward compatibility: ISO 20022 is not backward compatible with older messaging standards like SWIFT MT (Message Type). This lack of compatibility can challenge organizations that must exchange messages with systems still using older formats. It may require additional conversion or translation steps to ensure interoperability between ISO 20022 and legacy systems.  Users of the new messaging system often need to maintain original ISO 20022 messages in their original format and create secondary link tables to be able to trace legacy messages back to their original ISO.
7. Governance and maintenance: ISO 20022 is maintained and governed by a consortium of financial institutions and market infrastructure providers. While this collaborative approach has advantages, it can also lead to challenges in decision-making, standards updates, and ensuring consistent global governance. Coordinating changes and managing versioning across different stakeholders can be a complex task. Changes and enhancements also require a majority of the governing body to approve, making it difficult for some regional needs to be addressed in a timely manner.

It's important to note that many of these flaws and challenges are not inherent to ISO 20022 but rather arise from the complexities and considerations involved in adopting a new messaging standard across a diverse financial ecosystem. With proper planning, coordination, and investment, organizations can mitigate these challenges and leverage the benefits of ISO 20022.