The Shimano Nexus 8 with foot brake would be another good option, except in Denmark (and some other countries) it was only possible to order the old SG-8C20 version. It seemed that Shimano refused to sell the SG-8C31 until the old model was sold out (on a per-country basis), and no one could order one for me, nor could I find any for sale online.
I also considered the SG-8R36 with a roller brake, but it was more expensive and we wanted to stick with a coaster brake (foot brake) unless we had a really good reason to change.
So we ordered the old model, the SG-8C20. It had probably been sitting on a warehouse shelf for years. I installed it myself. Right away I had some issues because the "cassette joint" wanted to occupy a bit of space which the chainstay was using. I installed some extra washers and bent the cassette joint a little to get the shifting in order, but the whole thing was always a pain to fiddle with.
The hub itself was functional, but made me nervous that it was headed for premature death. From day #1 it made various clicks and had various shifting issues between gears 4 & 5. Gear 5 (direct drive) seemed like it might slip at any time (but it never actually slipped, except after shifting from 4th). Adjustment of the shifter could address the various issues but always at a cost somewhere else. At some point, the clicking and humming alone was enough for me to know what gear my wife was pedaling in, if I rode along behind her. Still, it was working.
Happily it seems there are ways to address these problems.
The Nihola company had started using Nexus 8 hubs (I presume SG-8R31), so when visiting Copenhagen I checked with them to see how they handled the cassette joint. Turns out they had some different non-turn washers that I didn't realize existed: 5L and 5R. They sold me a set of them for a reasonable cost, and one source of irritation dissappeared.
On my next visit to Denmark I took the Nexus 8 into Byman Cykler and had them put some new grease in it. Now this was the magic the hub needed. Most of the noises and shift problems vanished and haven't come back (in over 4 months). So, moral of the story: hubs like fresh grease.
There is one lingering irritation with this hub, however. It works its axle nuts loose over time, and then pulls itself crooked in the frame. I guess I'm putting too much torque on it. Always carry a 15mm wrench!
So now I can make a bit of a verdict on the Nexus 8 with coaster brake as a hub for a Nihola. Its pretty good. A bit rough in 3rd and especially 4th gear, in fact its generally a rough hub. Buts its strong: ours is set up with a 24t sprocket on the hub and 38t on the crank, well beyond any ratio any hub company endorses, and I've put my entire weight on it countless times, plus added arm strength. I've gotten small blisters from pulling on the handles to force the pedals around. (I do these terrible things in 1st gear exclusively, which is probably the strongest gear available.)
The Nexus 8 is a strange hub. Its not symmetric (i.e. the top ratios are not mirror images of the bottom ratios) and the least efficient gears are in the middle of the range (3 & 4) rather than the ends. However this works out well enough on a Nihola. For any real hill, I just drop to 1st gear and pedal. On any reasonably flat ground, I can stick to gears 5-8, which are mostly pleasant. I never make any strenuous effort in 3rd or 4th gears.
The foot brake is strong enough to slide the rear tire at will, though Oslo hills might cook the hub. More than once I have been unable to touch the hub's shell after a good downhill. After that happened a few times, I decided to have a V-brake installed in addition to the foot brake, so now that Nihola has two rear brakes. Overall I am very pleased with the foot brake. I'm convinced foot brakes are the best rear brakes for Niholas, except possibly in deep snow, ice, or mud.
But is the Nexus 8 a worthy upgrade over a SRAM P5? For any moderately serious cyclist, I would say yes. The ratios are closer, which makes it easier to go fast without knee injuries. The lowest ratio is good and low. The hub itself is stronger. It shifts easier. Its easier to handle a freezing shift cable. There is no "clickbox" sticking out in the way of pannier bags. On the other hand, the P5 is a lot smoother in operation, its likely a bit more efficient, and it doesn't take tools to disconnect its shift cable.
Anyway, Nexus 8 is a good Nihola hub.