A Shimano Nexus hub, for instance, has an MTBF rating, a mean time between failure period, which we normally take as its service life because the thing is so cheap that it makes more sense to replace it with a new one than to repair it. I can't remember whether the Shimano Nexus MTBF is 30K or 50K (or both, miles and kilometers). It doesn't matter. The facts are that it is a less capable box than a Rohloff, and generally speaking isn't expected to last as long.
Or £235 will get you a Shimano Nexus gearbox, which I admire, even though I wore out two before 5K.
My Shimano Nexus Premium 8 hubs cost substantially less than £235, as I had to buy them online (not available locally here in France). I got mine from Germany.
I currently have 3 Nexus hubs in service, an 8-speed on my Thorn Raven Tour utility bike, an 8-speed on my wife's bike and a 7-speed on one of the visitor bikes.
AFAIK, the Shimano Nexus series were, and maybe still are, vulnerable to water entering through the large right hand bearing. These hubs are grease-lubricated from the factory, so do not normally have the oil-flush feature of the old Sturmey Archer 3 and 5 speeds. There are several online accounts of Nexus 8 hubs destroyed by water entry when used in wet conditions.
I remove the internal from the shell when receiving new Nexus hubs, lubricate the external bearings with a generous amount of outboard motor grease and dunk the innards in synthetic oil. I repeat the process about once a year.
This invalidates any guarantee and is slightly more hassle than the Rohloff oil change, but has so far given good results for me. Nexus hubs seem to run more efficiently with oil rather than the factory grease.
Online reports suggest that Nexus hubs will run for a long time if protected from water entry and if the cable is kept properly adjusted (the latter is much more critical than for a Rohloff). If I get around 5,000 kms from them I will be happy.
Nexus 8 premium and Alfine 8 hubs have some of the same advantages over derailleurs as Rohloff - low maintenance, relatively clean and easy to operate by someone who doesn't want to learn how to use two gear shifters. The 3 most often quoted disadvantages of a Nexus 8 (and most other hub gears) compared to derailleurs are:
- less gear range. Not an issue for me for local use, but would be for touring mountainous areas.
- lower efficiency. I did some timed rides after converting my old mountain bike from 3x7 derailleur to Nexus 8 Premium and noticed hardly any difference. Factor in the time gained by doing less transmission maintenance and Nexus 8 premium is faster. The older Nexus 7 hub I have is noticeably less efficient, so is currently on a visitor bike only used for fairly short rides.
- weighs more. The difference is about the same as a Rohloff compared to a derailleur transmission, a few hundred grams. Not an issue for me.
As André states, they are cheap enough to replace when they fail. I recently bought two more Nexus 8 premium hubs, one is due to go on a projected "new" bike that will replace the oldest of my two visitor bikes (a 3-speed that dates from the 1970's) and the other will probably replace the Nexus 7 on the newest existing visitor bike.