The design pattern is based on convention only. This means that a developer is free to stray from the convention whenever they want. In a complex app that many developers work on concurrently, it is very likely that at least one of them will stray from the convention at some point.
In comparison, a class based UI framework like UIKit on iOS forces all developers to stick to using a standard set of APIs to customize views. IMO this makes code way more predictable and this also makes it much more maintainable.