Pro Extended predefined classes
Zend has a large library of predefined classes with which developers can create maintainable and stable web applications. This is done if the developer works within the constraints and with the components of these different predefined classes, which makes the application more maintainable.
Pro Allows customization
For developpers not used to Zend, they can use a predefined structure and use preloaded components and classes to build and maintain their application. But for advanced developpers, they can customize the structure to stick to their needs (or their likings) and extend primary Zend components for fine-tuned apps or replace the initial predefined Zend components by third parties components such as Doctrine ORM or another logging or templating framework.
Pro Easy to build an API with Apigility
Apigility is a collection of Zend Framework modules. It's useful for building the API architecture of a web app quickly and painlessly by providing a flexible engine.
Apigility also has a web-based UI that allows developers to quickly create and modify API services, configure authentication, set authorization rules, set up validation and write new documents.
Con Loose documentation
Since Zend has a loosely-coupled architecture, it means that the documentation will be quite loose as well. Even though there's a lot of documentation for the framework, it's still hard to use it as a guideline to create a completed project. Although this can be less of a problem considering all the tutorials and guides out there.