Mid drive = more torque, as they can use the bike's gearing. Great for climbing, but they apply a lot more torque to the chain & sprockets, which (so far) are not beefed up for this duty, so it means more frequent maintenance.

Rear hub drive = cheaper, simpler, gives the option to motor home in case of chain breakage. Rear wheel is what has more traction under acceleration, so this makes sense from a performance standpoint than the front, however, it leads to rear-biased weight distribution, which is no good for bikes that will be airborne or carried on lighter duty car racks.

Front hub drive = simpler, since it's separate from the mechanical drivetrain of the bike, however weight shifts away from the front wheel under acceleration, so they tend to spin out easier, which will be bad on loose surfaces or uphill. Easier to retrofit.