You can't/shouldn't shift under load with a derailleur either. Pinion gearbox has an option of 18 gears with a 600%+ ratio while something like SRAM Eagle 1x12 has 500% ratio. I don't know about the efficiency but i think something like this should be the future - https://www.pinkbike.com/news/allen-millyard-part-1-2008.htm... or maybe there is proper reason why nothing like this has come into the market - shifting without pedalling or under load, closed system (don't care about dirt/water) and no noise.