query type 'nid' or 'oid' can cause clashes if you are indexing multiple installations with the same node or object ID. This obviously results in unpredictable moreLikeThis results, since Solr simply fetches the first record that matches the object or node ID.

You can see this in ezfeZPSolrQueryBuilder::buildMoreLikeThis(), as the queries built are simply:

        $mltVariant = 'q';
        switch ( strtolower ($queryType) )
        {
            case 'nid':
                $mltQuery = eZSolr::getMetaFieldName( 'node_id') . ':' . $query;
                break;
            case 'oid':
                $mltQuery = eZSolr::getMetaFieldName( 'object_id') . ':' . $query;
                break;

$mltQuery should, by default, automatically append the installation ID of the current site. Also, the moreLikeThis fetch function should support an extra parameter for the installation ID.