rules_set_registry: only stores schema itself, for example {'type': 'string', 'required': True}
schema_registry: stores complete schema, for example {'name': {'type': 'string', 'required': True}}, it can be used directly.

It's not easy to allow_unknown for parent schema (for nested dict), because you cannot put allow_unknown inside the defination of schema. You have to change the allow_unknown attribute for the instance of Validator.