Mid-Drives are very good for low wattage motors. But over 500 watts can put too much stress on bicycle drive chains, especially if it is a narrow chain for more than 7 speeds. BMX single speed chains are the best if you don't want to use a motorcycle chain on the other side of the drive wheel. And slow acceleration is easier on the chain.
Direct drive hub-motors are set at a very high gear, therefore are not good for hard acceleration like in stop-n-go city driving. Even if they are Brushless motors and can be pushed past the peak efficiency wattage for a few minutes, they need more ventilation.
Geared hub-motors are not available in more than a 1:5 gear reduction. Hub-motors can obtain full thrust at a slower speed than most other motors. Most hub-motors do not ventilate well, so they have heat problems when using high amperage while climbing hills in a gear that is too high.
High speed high voltage motors are better than slower motors because they do not build up so much heat. And are more efficient than hub motors, even with the large amount of gear reduction needed to produce the amount of thrust needed.
Two speed transmissions are all the gear ranges needed for electric motors, unless you want a very wide range of speed, or if your using a high speed motor that does not have enough low speed thrust. ie: your government may not allow powerful enough motors for your needs. If you are allowed all the power you will need, but just need to cut it off at the legal speed, that can be done with the controller. If you live in very steep hill country and you're going to be moving a lott more weight, you may want three gears.
Two motors in tandem can pump-up the output thrust without increasing the overall speed that is set by the fixed gear ratio. A controller for each motor is needed but then you can program them to power-up the extra motors when needed. And both controllers can be wired into one accelerator.
What are the advantages? Electric motors loose thrust ability when climbing hills at a slow speed; if you want your high speed gear-ratio to be set at about 20mph and you need a 10mph gear ratio to get enough thrust to climb your hill with only one motor, a second or third should provide enough thrust at 20mph without having to use a larger motor. The bigger motor would take more amperage to keep running at a high speed when cruising on flatter land, where the extra power is not needed. Also motors made for high speed are more efficient than slow speed motors; less energy is turned into heat.
It would be possible to use a gearless hub motor for a cruising gear with a mid-mounted motor with a large gear reduction for hill climbing.
Two Separate Reduction Ratios
I like the idea of using two motors separately. One with a large gear ratio reduction and the other with a higher speed reduction. Then the controllers set to turn on the slower gear reduction when needed and the other to turn off. Making it a two speed machine with the same legal wattage.
Vector Control (field oriented control) is a better way to make motors produce more thrust at lower and higher speeds than using gears. With controllers having more computing power now, they can control a motor much better than ever before. It requires a three-phase AC motor and a more expensive controller. These would be very good for a motorcycle that needs a wider range of torque-thrust than a simple electric bicycle.
Increasing the number of phases above three allows the stator MMF's to be shaped so that the motor produces significantly greater torque. http://commutercycling.blogspot.com/2016/05/vector-control.html
E-bike laws should be changed to reflect the gear speed so that you can still have enough power to climb your worst hills. i.e. Washington state allows only about 1000 watts for a power assisted bicycle and 1500 watts for a moped. Well if you want to move 600lbs (272 kg) up an 8% grade you are going to need a gear ratio that will drive at about 12mph (19 kph) at the r.p.m. that your motor will produce at that 1500 watts.
One reason the laws don't let you have more than “a given power output”, is because even if you have a controller set to turn off at a given speed, the sensor can be by-passed. And it is very unlikely that a law restricting the bike to the use of controllers that do not work without speed sensors, will be functional in the USA.
Some states require that the crank be powered as well as the drive wheel. And others require a torque sensor on the cranks. This is probably the only way to restrict the speed that cannot be bypassed easily. But this would also need to be examined by an “engineer”.