Which would prevent data from appearing in a Custom Report?

A filter that filters out all data will prevent any data from appearing in a Custom Report because the report will have no data to display after the filter is applied. Similarly, using dimensions and metrics of different scopes (e.g., a session-scoped dimension with a hit-scoped metric) will prevent data from appearing. Google Analytics cannot combine data from different scopes in a meaningful way within the same report, leading to an empty report or an error. Not sharing the Custom Report with other users would not prevent data from appearing in your view of the report; it only affects others' access. Too many dimensions applied to a Custom Report might make it difficult to read or cause sampling, but it would not typically prevent data from appearing entirely unless combined with other issues.