All the gears on an internal hub are usable, there's no overlap. These days the ubiquitous Shimano Nexus hubs can shift under load. For the kind of bicycle most people buy they are a reasonable alternative because there isn't a fragile derailer, there's no double shifting, and the adjustment is static alignment of two yellow marks.

In terms of friction, there are no jockey wheels and the chainline can be perfectly straight. Anyway, the internal friction of a geared hub is going to be unnoticeable for an average cyclist because the tires will be under-inflated, the chain covered in crud, and the brakes will be rubbing on untrue wheels.

But like everything it's a tradeoff.

Good internal hub gears can be shifted under load, and the Rohloff one is probably as good as it gets.

I ride a Rohloff and you do not shift them under load. Most of the shifts will tolerate a light load, but going from 7-8 while steaming up a hill will immediately bind the gears and you need to backpedal a few teeth to get it unstuck. By the time that's all sorted, you've lost your momentum and need to downshift again, not to mention taken months off of the lifetime of your very expensive hardware. As you can probably tell from my description, this is a mistake I've made. Binding up your brand new $1500 transmission is a great lesson to never do that again.

The Rohloff can not be shifted under load. You can shift by quickly letting off power, shift and then resume like you can do with other planetary gear systems.

Here's a discussion on the subject of shifting under load, with opinions from "both" sides:

http://thorncyclesforum.co.uk/index.php?topic=4172.0