I don't doubt they are inefficient but I can't say I've noticed the difference on a pedelec. There will be one gear that is a direct connection and as efficient as derailers, this is usually the gear you cruise in and has been set up like that on my 906.
Direct drive is usually the gear nearest the middle and always so on Shimano hubs. Bikes are usually set up for cruising in higher gears or top gear where the inefficiency is much greater, and that's essential with chain drive bikes if they are to be legal.
Derailleurs are much more efficient, typically 97 to 99% if in good condition, and contrary to popular belief, the chain running out of line in some gears has a minimal effect.
Hub gear efficiencies vary from 100% in the direct drive gear down to as low as 84% at the gearing extremes. They are reported as an average of the efficiencies in each gear including the 100% direct gear, so a 3 speed hub benefits much more from that than an 8 speed.
Manufacturer's figures are rubbish, independent tests much more reliable though not always so. The highest independent average seen from one tester is 91% for a 3 speed SRAM hub, but other independents have disputed that and criticised some aspects of the testing. You can safely assume at least 10% lower efficiency than derailleurs from most hub gears.
One problem is that hub gears use epicyclic gears which, using three cogs as they do for each power transfer, as much as triple the losses of a single engagement. In addition, these cogs are small diameter so the tooth engagement approach angle is greater and so generates higher friction. Worse still, to generate more gears from fewer epicyclics, they use them in combinations to achieve all the ratios, so at each combination the losses are doubled by the compounding.
Since these losses grow with greater gear range, newer hubs with 8 or more gears are made larger diameter to enable larger cogs and keep losses within bounds, Shimano's 8 speed hubs and SRAM's 9 speed i-motion hub being examples of this.
The direct drive gear position varies, Shimano and SRAM hubs near the middle excepting the Shimano 7 speed which has no direct drive, all being indirect. The Sunrace-Sturmey 8 speed has direct drive on bottom gear, so has to use small chainwheels and larger rear sprockets to keep the gear ratio increase with changes up within bounds.
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