Mid-drive motors were designed to improve upon a number of shortcomings found in hub motors.
The single largest advantage that mid-motors have over hub motors is their gear ratio. They allow the rider to power the rear wheel via the same chain and gear set as the pedals, which means that a low gear can be selected for powering up steep hills or accelerating from a stop with massive torque. A mid-drive motor in low gear can climb steeper hills than a hub motor of similar power, and can climb hills for longer than a hub motor, which could overheat on long steep hill climbs.
A mid-drive motor is also usually smaller and lighter than a hub motor of similar power. Bafang recently debuted a new line of mid-drive motors for racing bikes that weigh only 5 lbs (2.3 kg). Smaller and lighter mid-drive motors are often stealthier because they can be incorporated directly into the bicycle’s frame. Many people don’t even realize that a mid-motor bike is an electric bike just by looking at it.
Changing a tire on a mid-drive motor e-bike is much easier, since you don’t have a heavy hub motor to deal with. You just change it out like on a normal pedal bike. Plus, since you can use normal bicycle wheels, you have the freedom to use any wheels, tires and cassettes that you wish.
Lastly, mid-drive motors allow the use of true torque sensors for pedal assist systems, which regulate the motor power based on how hard you push on the pedals as measured at the crank. Hub motors often rely on cadence sensors for pedal assist, which only regulate motor speed based on pedal speed, and can cause jerky or awkward motor timing, especially when hill climbing or moving the bicycle around obstacles.