Do not use Guvnor in the following scenarios
a) If you have business rules in a database as a part of an application and no new application is needed to manage them. This is a scenario where rules exist as a part of an existing application and there is no need fot future expansion.
b) If you already have a rule management system tailored to your environment, migrating to Guvnor may not be necessary. But at the same time if you think you need more flexibility, expansion,control on business rules in future, then plan for migration.
c) If you are using rules/business logic to solve complex algorithmic problems and they are integral part of the application, don’t go for guvnor.
So the conclusion is, first try to understand your application requirement and then select the proper tool.Otherwise some time people use different tools like Guvnor, rule engine without any proper justification. As a consequence, proper advantages are not realized and the application does not get any benefit.