The great gear guru Frank Berto dismissed internal gear hubs a 'friction boxes'. Imagine his surprise when years later he ran an instrumented test and found Sturmey and Sachs three speed hubs overall the most mechanically efficient multi-gear systems.
I've not seen a study or test showing an IGH outperforming Derailleur systems as far as energy in/energy out efficiency. If any did, it seems racers would be using them. I'm open to learning more.