The TMDb API links mostly to YouTube trailers. Of the ones I tried, 1/3 were online but 480p at best, 1/3 were simply offline, and 1/3 were unavailable due to country restrictions.
So, yeah, it's pretty bad. We've also looked into http://www.traileraddict.com/trailerapi but there doesn't seem to be any reliable way to get only trailers from there (you often have DVD bonuses, making of, etc.) and their embed is pretty messy.