A french company is developing similar technology called PERS (pedaling energy recovery system) [1] that is actually chainless. This comparison [2] claims an overall efficiency of around 64%, with both generator and motor losing about 20% each. They were interviewed by the Laidback bike report [3].

[1] https://www.cixi.life/pers-technology

[2] https://www.ebike24.com/blog/chainless-ebikes

[3] https://www.youtube.com/watch?v=DyGDZw3VPlI