The UIViewController Spaghetti Monster
This is the largest pain point I face when doing iOS development.
“UIViewController is the centre of the iOS world. Apple’s tools like Xcode and Interface Builder, and the workflow they encourage, put the view controller at the centre of everything. The default place to start when you’re adding new code or features is the view controller. When you add a new screen to your app, all you’re given is a UIViewController subclass and a lot of helpful methods to shoot yourself in the foot.
Because of this, code and logic tend to gravitate towards the view controller. The view controller ends up being the delegate for a large number of different objects. It ends up doing view setup and layout, it ends up dealing with core data, table view datasource/delegate methods, and a huge number of other responsibilities.
Apple has tacked on a large number of “convenience” methods which let you directly access the enclosing containers like navigation controllers or tab bar controllers. This makes it very easy to couple the view controller to the specifics of your app’s navigation, and to the other view controllers it interacts with. Storyboards also contribute to this problem. This makes it difficult to reuse the view controller in other contexts or applications.”
Mike Weller's post is a great explanation of why writing reusable UI code in iOS is very difficult.