2. We often forget about legacy data or de-prioritize the impact of changes in data management practices. RDA brought about many data changes. While most of those are not significant as they do not deal with formatting of data, some do change the expectation for data.

https://twitter.com/georgiawebgurl/status/1592215750000918528

3. Any variation in expected data (e.g., 260 vs. 264) can cause issues. An 007/008 expectation for media or 336-338 fields which may not appear in older records. You can't build facets and search parameters on what doesn't exist.