A gearbox hub sounds to me like "No User-Serviceable Parts Inside".
The Rohloff hub is exactly that but it seldom goes wrong.
There are hubs out there with tens of thousands of miles on them still working perfectly.
One downside is that they are comparatively heavy and all the weight is in the rear wheel still you save the weight of the second chainring and a number of links of chain. Or you could go belt drive which is much lighter than a chain (and you can't use a belt on a derailleur system!
I am biased because I own two!