I think the biggest challenge doesn't really have to do with the bearings. For a hub motor were not gonna be able to just take an off the shelf motor and bolt it on. The typical outrunner has the can supported by the shaft on one side. With a load on it, the magnets are going to run into the stator no good.
For a hub motor your going to need a whole new can. It needs to be supported on both ends, and if you want to make it universal you need to have an adapter that will connect the stator to the axle, allow the wires to pass thru the inner race of the bearings, and keep the stator from turning about the axle. Then your going to need much larger bearings. I would guess 12mm or so at least where the wires pass through, plates to attach the bearings to the can and a good way to attach rubber to the can. You might also need a way to keep the motor cool.
Its definitely more work than just larger bearings, especially keeping it in a nice small package. The guys that have done it already have overcome some challenges for sure.
