In a custom framework I wrote, any JS call to tb_show throws an error when MP6 is enabled:
Uncaught ReferenceError: _ is not defined
Specifically on line 50 in svg-painter.js
I added the following line to mp6.php on line 160 to fix the problem:
wp_enqueue_script( 'underscore' );
I would normally just include it myself in functions.php etc, but since svg-painter.js uses underscore, I figured it should really be added there. Just posting this if anyone notices the media uploader not working as expected.