In a new post to his site Frank de Jonge makes a distinction between packages versus components, pointing out that components are always packages but packages are not always components, and what it really boils down to is a problem of dependency.

The PHP landscape has fully transitioned into its Package Age™ [...] However, due to PHP's nature, there are some problems. While packages are great for re-use outside of frameworks, dependencies are still an issue. Namespaces resolve conflicts between classnames, but they do not offer a solution to package versioning. Especially in a framework-context, this can become very problematic. A real-world-example for this is Guzzle.

In his Guzzle example he describes the main problem - when packages restructure or make changes incompatible with prior versions and dependencies conflict and both must be installed. He also points out that, while this is bad for just packages, it can be made even worse working with components (his name for framework-based packages). Problems he mentions are the previously mentioned dependency conflicts but also some unexpected quirks with how Composer chooses to install packages. He gives an example of this second one with the installation of the Symfony EventDispatcher component and how, upon closer inspection, Composer seems to be installing two versions of the library at once.