Some YouTube videos impose restrictions on which websites are allowed to embed them. This appears to be done by checking the content of
the Referer header and/or accessing the
iframe's parent. Because this add-on loads videos dynamically, some browsers such as Safari anonymize the origin of the request. In response, YouTube may refuse to serve the video and display a "Video unavailable" error instead. As a workaround, an option to use YouTube's default embed as been added. It will disable the lazy-loader and the miniplayer and it will allow YouTube to gather info about the parent webpage.
TL;DR: if YouTube videos don't load on Safari, check the
"Use the default YouTube embed" option. Otherwise, don't.