Going to list here missing config.yml validations that I encounter. Feel free to add more.

• Unique fields names fix(config): validate unique fields names under the same level #3804
• Widgets specific field configuration feat: add widgets schema validation #3841
• Unique file names (Warn on duplicate collection, file, and field names in config #1035) (fix(schema): enforce files and collections names uniqueness #3913)
• Unique collection names (Warn on duplicate collection, file, and field names in config #1035) (fix(schema): enforce files and collections names uniqueness #3913)
• Backends configuration (missing repo, app_id for implicit auth, etc.) Related Editor friendly error messages #1693