This is a very nice concept, but works only for single speed bicycles...
I have a gear hub [Shimano Nexus redline 8] and would Never Ever replace it with anything else.. This solution might be able to kill the joy of riding a bike and might turn the bike to a vehicle which takes you from A to B
About sweating: I ride every day, and still sweat. I am unable to ride my bicycle 'slowly' or in a 'comfy way'.. So I rented an office where there is a bathroom..
What I like is that it seems this is very lightweight, related to other e-bike solutions/kits.