I suspected that there might be some "interesting" road surfaces in Brazil, but didn't want to assume.

I wish someone with riding experience will comment, but this is my understanding:

Curbs are a particular problem due to the low ground clearance at the "steering column" location. If that control horn drags the ground as you pass over the curb, you will certainly fall. So you need to go quite slow, just in case, and going slow makes it much harder to balance.

To balance any bike you have to keep steering the wheels back under the weight whenever the weight starts to tip. There are various effects that allow the bike to do a lot of this steering for you, but most don't help much at very low speeds...more trail being the exception.

An unloaded bakfiets has most of the weight (the rider) at the back. So when you start to tip, you turn the the front wheel to correct. But at first this doesn't help much, because there is not much weight for the front wheel to move under. Only when the whole bike is turning does the back wheel begin to move sideways and come back under the rider. Due to the long wheelbase, this delay is much longer than on a normal bike, and also due to the wheelbase you can't turn very sharp, so there is a real limit to low speed stability...if you get too far off balance at low speed, you can't turn quickly enough to avoid falling over. This is much less an issue on an empty long-tail, because the long wheelbase doesn't much change how fast the front wheel can move from side-to-side to restore balance. In fact you are, percentage of wheelbase wise, much closer to the front wheel on a longtail...you can easilly balance, and the bike as a whole never much changes direction as a result. "Like it is on rails" is how riders discribe this. If you put a load on the back much heavier than the rider, though, you run into the same issue...but you don't have a steering linkage hanging down in harms way, so most of the time you don't need to go so slow.

The steering geometry used on a bakfiets adds to this issue. So that the steering is not too heavy when loaded, the bakfiets typically has lower trail, and this makes it less self stabilizing especially at low speed which adds to the problem.

Bakfiets designers have a compromise to make: They can make a large stiff structure connecting to the front wheel, but this adds weight and takes away cargo space. As a result, the bike ends up maybe being stiff enough, but maybe not, and with not much extra margin for unusual loads.

I think another issue with classic bakfiets is that the rider can't see the front wheel, so is often suprised by the timing of when it contacts the curb or other hazard. This probably improves with experience...just as a good car driver will instinctively know where their front wheels will run when driving a familiar car.

When weight is added to the box, both of these issues are reduced. Now when you start to tip, you turn the front wheel, and since there is now significant weight for the front wheel to move under, balance is quickly restored.

Also, with normally only a 20" front wheel, curbs, potholes and suchlike are more of a factor for the smaller wheel...much as even a small stone can stop a skateboard wheel. If building for rough roads, I would consider taking an unconventional route, and using a 26" front wheel...it makes the bike only a few cm longer for a given box size. You could also design for trail more like a normal bike, and accept that the steering will become rather heavy with bigger loads. You might also want to add a bash guard around the steering linkage that will prevent it from catching on rough roads. It will be bad to drag the bash guard, but not so bad as to drag the control horn and crash when the front wheel suddenly turns full to one side.