When are categories bad/dangerous?

I'm using categories with Core Data. Some of the tutorials I've read and lectures I've listened to say that categories are often considered "bad" practice. But because Objective-C is so dynamic it seems perfectly okay to define methods somewhere else, especially because only public properties of a class can be used. What are the pitfalls I should be looking out for when using categories? Or is there some reason that categories are actually bad practice? The reason I'm using them with Core Data is so that I don't have the rewrite my add-on methods every time I regenerate the subclasses.

The only "danger" I can think of is when you use them to replace methods in the original class rather than subclassing.

When doing this you lose the ability to access the original implementation, which, since it is usually a private method you are overriding, could have unforeseen effects.

Using categories to add extra methods to any object of a particular class is great, and precisely what they are for. Using them for core data, as you are doing, is fine because it does allow you to change your model and regenerate the "vanilla" object without destroying any extra code.

Tip of the hat to @CodaFi for this bit of documentation from apple:

Although the Objective-C language currently allows you to use a category to override methods the class inherits, or even methods declared in the class interface, you are strongly discouraged from doing so. A category is not a substitute for a subclass. There are several significant shortcomings to using a category to override methods:

When a category overrides an inherited method, the method in the category can, as usual, invoke the inherited implementation via a message to super. However, if a category overrides a method that exists in the category's class, there is no way to invoke the original implementation.

A category cannot reliably override methods declared in another category of the same class.

This issue is of particular significance because many of the Cocoa classes are implemented using categories. A framework-defined method you try to override may itself have been implemented in a category, and so which implementation takes precedence is not defined.

The very presence of some category methods may cause behavior changes across all frameworks. For example, if you override the windowWillClose: delegate method in a category on NSObject, all window delegates in your program then respond using the category method; the behavior of all your instances of NSWindow may change. Categories you add on a framework class may cause mysterious changes in behavior and lead to crashes.