See also: Inspection of free version of Hummingbird. This review focuses on the the Pro features.
The general findings are applicable: The plugin has multiple modular features that may attempt to modify server or website configuration. There may also be conflicts with any existing site functionality. For example, plugin caching may conflict with other website caching in place. Therefore, the recommendation remains to select features carefully and test on non-production infrastructure.
Ensure that enabled features are desired. For example, the cleanup feature will remove draft posts.
Reason for the 'Use with caution' result
The plugin has been given this recommendation at the tester's discretion:
No critical issues were found in reviewing the Pro version features. The plugin is marked ‘use with caution’ due to the possibilities of conflicts with existing server configuration and features (such as caching) and to advise due caution if enabling features such as cleanup that may remove content.