The bold part is by design, more or less. Embedded videos replace the original text with a block element, the bold BBCode is meant to apply to text so it disables block elements that don't display the original text. I think that makes sense.
The spoiler BBCode should be able to display videos,
I'll look into it asap. Actually I just did, it won't display it because the spoiler markup uses a text span (for inline spoilers) rather than a block element. It means it's not supposed to contain other block elements. I don't know what to do with that.