It seems there is fair use senarios regarding copyright. From what I can gather to sample a few seconds is reasonable but to play the whole video would seem not.
This would make embedding a video created by someone else, without permission, difficult as the whole video is played, even if the source is credited.
Secondly, if one was to make Steem Dollars from the post would one be obligated to pay royalties etc? This would be difficult if not impossible in many cases.