My understanding is that the efficiency loss is strictly limited to delivering electrical power back to mechanical power via motor instead of via the chain drive. The problem with this design is that it completely eliminates the ability to deliver mechanical energy directly. The rider pedaling charges the battery which then powers the motor. Going from mechanical energy to stored electrical energy is going to introduce a much greater loss in efficiency than a mere 5%. In a more standard e-bike model, when the rider pedals, my understanding is that they are delivering mechanical energy directly, bypassing the conversion to electrical energy. I may be wrong as I am not very familiar with e-bikes, but, if this is the case, the 5% figure seems highly misleading.