How you use a factory matters. A factory that sits as a static function inside the same class it's creating is one thing. It may just be a nicer interface to instantiate the class than the constructor itself.
On the other hand, it may do something with the object it's instantiating before passing it back. Such as putting it into global scope, or tying it to something outside its chain. Factories that sit outside the class file are often prone to all kinds of grotesque later mucking-with that causes the instantiated object to bind to other scope, and never be properly garbage collected. That's what I'm talking about.
Build better constructors and you won't usually need factories. If you need factories as an API, build them next to the constructor.
On the other hand, it may do something with the object it's instantiating before passing it back. Such as putting it into global scope, or tying it to something outside its chain. Factories that sit outside the class file are often prone to all kinds of grotesque later mucking-with that causes the instantiated object to bind to other scope, and never be properly garbage collected. That's what I'm talking about.
Build better constructors and you won't usually need factories. If you need factories as an API, build them next to the constructor.