search in node-with-comments plus in unpublished nodes
You can use 
hook_searchapi_entity_property_info_alter:

function mymodule_searchapi_entity_property_info_alter(&amp;$info) {
    if (isset($info['node']['bundles'])) {
        // For each content type.
        foreach ($info['node']['bundles'] as $bundle_type =&gt; $bundle) {
                $info['node']['bundles'][$bundle_type]['properties']['comments'] = array(
                    'label' =&gt; 'node comments',
                    'description' =&gt; 'the comments of the node',
                    'type' =&gt; 'text', // or whatever you want here
                    'getter callback' =&gt; 'mymodule_searchapi_get_whatever', // name me whatever you want

                );
            }
        }
    }
}


and your callback

function mymodule_searchapi_get_whatever($entity, $options, $name, $entity_type, &amp;$info) {
    // return the comments here, as you have everything in the function variables - i.e. use return.
}


Look at example implemntations of this hook and it should be enough to get your started.