createDefaulted does not specify a supplier to the default value, of a defaulted registry. Hence it is unusable because vanilla fails if there is not default entry.

EDIT: Upcraft clarified that any registry has a default value, so it applies to every custom registry

update:
FabricRegistryBuilder is fine, so long as registries are only created AFTER the vanilla Registry class has been fully loaded.

Not applicable for most cases where people create registries after registry class has been loaded.