Your function's arguments are wrong. See hook_form_alter docs.

So basically you need to rewrite your function as follows.

First, grab the correct form ID.form_alter functions don't have a $node but in some forms, full node object form is in $form. So use the following code. You will now see at least 2 message on your messages area with form ID and array explanation of $form.

function uc_button_form_alter(&$form, &$form_state, $form_id) {
  drupal_set_message($form_id);
  drupal_set_message('<pre>'.print_r($form, 1).'</pre>');
};

Now, check if the node ID you are looking for is available in the $form array. (in most cases, it's in $form['#node']). Now you can change the function to the real code.

function uc_button_form_alter(&$form, &$form_state, $form_id) {
  if (isset($form['#node']->nid) && $form_id == 'edit-submit-' . $form['#node']->nid) {
    $form['submit']['#attributes']['class'] = 'node-add-to-cart';
    $form['submit']['#attributes']['xxx'] = 'xxx'; // we don't want  to clear existing values.
  }
};

However, even this implementation is not necessary. You can make use of default classes with tricky nesting if you only want to add a class to the button.

Update after 3rd comment: Try this: function uc_button_form_alter(&$form, &$form_state, $form_id) { if (isset($form['nid']['#value']) && $form_id == 'edit-submit-' . $form['nid']['#value']) { $form['submit']['#attributes']['class'] = 'node-add-to-cart'; $form['submit']['#attributes']['xxx'] = 'xxx'; // we don't want to clear existing values. } };