Rolling over the chips it makes? Does it automatically keep the bit perpendicular to the surface? Varying traction depending on the surface and debris on the surface being cut. Power cord getting hung up on an edge. Rolling off an edge would limit how close it could cut to the edge of the material it is on top of. Bits trying to deflect or "steer" as they cut through hard materials.
A cool idea. Nothing I'd invest in though. Too many challenges to overcome and laws of router bit physics to break.