I think, I found bug in Form functional:

<?php
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;

class TestForm extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        // Ok
        $builder->add($builder->create('text_test', 'text'));

        // Error 
        $builder->add($builder->create('button_test', 'submit'));
    }

    public function getName()
    {
        return 'test_form';
    }
}

Error:

Expected argument of type "string, integer or Symfony\Component\Form\FormBuilder", "Symfony\Component\Form\SubmitButtonBuilder" given

It is because method \Symfony\Component\Form\FormBuilder::add() contains check for instanceof self and any button-like type inherited from ButtonBuilder.

Both builders (FormBuilder and ButtonBuilder) have interface \Symfony\Component\Form\FormBuilderInterface.
So, if we change check of $child in method FormBuilder::add() to instanceof \Symfony\Component\Form\FormBuilderInterface all will be fine.

I checked this situation on 2.3 and 2.6 versions. Both have a bug.