How do I know when a render array will be processed properly?
See Render Arrays in Drupal 7


  Note: While render arrays and arrays used by the Form API share
  elements, properties and structure, many properties on form elements
  only have meaning for the Form API, not for the Render API. Form API
  arrays are transformed into Render arrays by drupal_get_form.


Because form arrays are converted to render arrays before they are rendered if you add a render array to the form array it will be also rendered when the whole form-array-converted-to-render-array is processed.

As the quoted text says you have to keep in mind that form arrays are not directly renderable, they have to be converted first to a render array. During this conversion, render array attached to the form array are bypassed because they are already a render array.