Longtail cargo bikes - why no internal-gear hubs? I really like the idea of ("compact") longtail cargo bikes, i.e. bikes with a regular frame shape but with an extra-large and sturdy rack for transporting medium-sized loads and persons/kids. Personally, I'd prefer one without electric assist. There don't appear to be many bikes to choose from in this category, I found the ones from Yuba, Xtracycle and Surly. All of these use a classic derailleur-based setup. I think that for such a bike, an internal-gear hub would be much more practical:


In inner-city traffic with a lot of stop lights, slow riders in front of you, pedestrians crossing etc., being able to shift quickly, possibly even under load, would be very helpful
The chain could be partially or completely covered, preventing entangling the cargo or passenger's pants
Less maintenance
Possibility of using a belt drive, even less maintenance


Why are there no longtail cargo bikes with an internal-gear hub? The only reason I can think of is that the ball bearings inside existing IGH's simply can't transfer the weight from axle to wheel, and:


The added weight doesn't matter much for such a heavy bike
Even though more mechanical energy needs to be applied for accelerating a bike with a heavy load, the rider's strength is the same, so it doesn't have to transfer more force in any instant (just for a longer time to get to a given speed as a regular bike)
I suspect the main reason is cost - internally geared hubs are more expensive than derailleur setups.

Torque capacity may also be a factor. You are wrong about the force the hub has to withstand being only dependant on the rider. The torque on the hub is determined by the gearing ratio and crank arm length as well as the force the rider can apply.

A heavy bike will require a lower gear ratio between the chainring and sprocket to lower the overall gear ratio range. A lower gear ratio means more torque on the hub input for a given force applied to the pedals.