Honda use to make a rear-tine, self propelled tiller that had half the tines going forward and half reverse. Shear pins would break in anything but the softest soil.
Counter-rotating tines would push the tiller back toward the operator, so if you hit compacted soil or a rock, the machine will come back at your feet. Likewise, most tillers that rotate in the direction that you are tilling will jump forward when you hit a hard object. I would rather have the machine jump forward.