What do you mean it is a bad idea? It's a bad idea because this part of Symfony is broken? Or it's a bad idea to do this ever? I do not understand what you mean here.

ArrayObject is specifically written to implement this use case. If we were not supposed to use an Iterator with ArrayObject, why would it implement IteratorAggregate?

We use ArrayObject throughout our projects because it gives us more control over type safety.    Are you saying that ArrayObject is not supported by the ResizeFormListener? If this is Symfony's official stance on the matter, then surely it should detect and throw and exception if one is used?