Yes, Mountain bikes are hard to ride. There are mainly three reasons why mountain bikes are hard to ride that is high rolling resistance, heavy bike components, and upright riding position. Mountain bikes are equipped with wide knobby tires, these wide knobby tires have a larger surface area that remains in contact with the ground. Large contact area means high rolling resistance and to overcome that rolling resistance you have to pedal harder. Mountain bikes offer upright seating positions. This upright position is comfortable but this upright position is less aerodynamic and because of this upright position air resistance increases. To overcome this air resistance you have to pedal with high intensity. MTB bikes are equipped with a lot of expensive components which increase the overall weight of the cycle and to keep your bicycle moving in the forward direction you have to pedal with more intensity