With a ball on a hard surface, surely the surface contact area would be reduced, therefore reducing grip and so making its use for directional control pretty much useless? It may as well be a skid! Unless its an inflatable ball that is - or not a true ball but have it contain a flat center region.
The ball would also need some sort of tread to allow it to expel standing water. With a wider shape, this could be more of an issue, increasing the tendency to aquaplane.
My Dad has a wheel barrow with a round ball, my Grandpa one with a traditional tyre. On soft ground the tyre cuts through leaving a furrow, the ball stays on top. Works nicely for a wheel barrow but in a plane I would normally want to cut through standing water. So for grass use only?
Also - if a hard ball was used, there may need to be changes to the oleos to absorb more of the ruts in undulating ground.
Just some more things to think about!