I think the pole in the center prevents your body from leaning forward more than the Segway will tilt (i.e. it can just tilt backwards, and push your body back

I think the pole in the center prevents your body from leaning forward more than the Segway will tilt (i.e. it can just tilt backwards, and push your body back where it belongs). With the Onewheel you can just lean forward as much as you like, and the board is supposed to accelerate to bring your center of balance back into the middle.