The main difference between any hub drive and any mid drive is a hub motor is basically a singlespeed, with all the drawbacks that entails. On a climb, even with equal (or more) power, it can struggle because its gearing isn't letting it apply that power efficiently. Like trying to accelerate a stick shift car uphill in 5th gear, or trying to pedal away from a stoplight in a high gear.
Mid drives put the motor at the same place you pedal, so the power goes through the drivetrain and you can control the gearing. So power tends to be applied much more efficiently.