If I'm the Secret Police in some oppressive state, then I just need to find out whether you seem to be using stego — which is naturally against the law, itself, and hence grounds for arrest. Then, I can use rubber hoses, bamboo splinters, the threat of violence against your loved ones, and what-not to "brute force" your passphrase.

If I'm the NSA, I just detect the presence of stego and stash the container for later — say, when my quantum computer finally works as advertised, or I can plant a keylogger or turn on the back door on the your computers and sniff your passphrase, or simply mine your social graph until I find some other means of compromising you.

The possibilities are hardly limited to a naïve, brute-force search across the set of (crypto algorithm, passphrase) tuples.

EDIT: But, to your point: yes, using video makes finding stego harder. It doesn't change the nature of the problem, though; it just changes its scale. Against adversaries with the computational power of a modern nation-state, however, if you're relying on scale to hide your behavior, licit or otherwise, you're only deluding yourself.

Me? I'm the entire population of the country. Which one of us is using stego?

To my reckoning, the search space would put the number of atoms in the universe to shame.