One of the problems with crank drive is that the rider and the motor share the transmission system,whether it be dérailleur or hub gears. Really annoying these cyclists they want to turn the pedals and input power!!!
We (Kudos) and Bosch soon learnt that chains are under a lot of load,especially up steep hills....although the motors are rated at 250 watt nominal they can exhibit 600 watts under load,a rider standing on the pedal can also input 400-600 watts. Trying to change gear,especially downshifts on steep hills,resulted in horrible graunching noises from the dérailleur or damage to the Shimano hub. In fact Bosch and Shimano had a warranty argument about crank motors plus rider power exceeding the power handling capacity of the Shimano Nexus hub....the result was generation 2 Bosch motor which turns down the power and delays the return of the power when executing loaded gear changes,a clever technical solution but results in loss of momentum,not an ideal solution uphill.
I had a customer at our Manifold Trail event in Derbyshire (lovely countryside) who was sure his KTM-Macina Bosch was not working properly....what was happening was that he did not change down quick enough,he was always in too high a gear and ultimately the bike came to a stop,you do need a technique to riding a Bosch crank drive up steep hills.
We overcame the problem by going to the Nuvinci hub,see our bike Kudos Eiger-Nuvinci,but it is an expensive solution,some customers love the Nuvinci ,it accepts the power and allows quick and wide range gear changes.
As a result of the foregoing we decided that overal the hub drive eliminated all the power transmission problems of crank drive and with the emergence of the Bafang BPM motor it also offered low speed grunt,the torque spread of that motor is much wider,invaluable at low road speed.
At this moment I can see no reason to use any other motor type,it does the job most of us require very well and at fair cost. I suspect,in the future, that some clever engineer will combine the BPM motor with an auto changing Nuvinci type hub as one unit that will simplify the e-bike installation but that is not yet available.
I do think there is a 'head in the sand attitude' to hub drives,I have customers who will only ride crank drive bikes and must be Bosch motor,they will not consider even test riding a hub drive bike but those who try a BPM motored bike are often amazed at the ease of use and performance,not to mention the average £500 cost saving.
KudosDave