I found the following issues with this plugin.
- double code the filter 'add_meta_boxes' in class-metabox.php
- class.frontend.php uses wp_reset_query in __construct. This is pointless since it can't be resetted. It also cause an notice.
Also on several other places you get notices.
Also I think there should be a way in code to disable a post type to get the meta box. I know you can remove it but you can't remove it from the list in your plugin. I know it just a small thing but it is confusing for users. Also attachment still can be disabled from the admin page but it is already disabled.