Sumo doesn't actually have all that many moves that can be used in a 'real' fight, does it? I've seen several martial art series where they try. But it generally ends up with palms, slaps, and stomps when they're down, without much else.
I would agree that if they can get you on the ground, you're in for a world of hurt. But it always seems too ceremonial and limited to be put against martial arts designed to take humans apart. Maybe in the ancient times? Was it ever used for actual combat outside the ring?