Motors mounted in the center of the bike can give more balance and weight distribution, with the weight lower down and in the middle of the bike. 

They tend to have higher Torque ratings because they have indirect power delivery which means they require more energy to achieve the same result as a hubmotor. They are also more complex and less easy to change in the event of an issue. 

They are a good solution for electric mountain bikes due to the balance and offer a comparable ride to hub motors for general leisure use.