The Nuvinci is, as far as I know, the absolute heaviest drivetrain you can install. You can subtract the existing drivetrain, you can subtract the front derailleur, subtract some chainrings, and subtract a few links of chain if you're really desperate to justify the hub, but even with all of those savings, it is still the heaviest drivetrain you're likely to find. If weight is up at the top of issues you need to consider for your bike, go with something else.
But I've been riding my bike for two weeks now, and I love it. I live in a 2nd story apartment, so that bike goes up and down the stairs at least twice a day, I definitely notice that the back is heavier than the front, but I have no problems carrying it. Of course the back is generally loaded up other gear, too, so it would be heavier with any hub. The shifting, if you can call it that, is smoother than anything I've ever felt. Up and down the stairs and lifting the bike unto the rack on the front of the bus are the only times I notice the weight of the hub. When I'm on the road, I only notice how much smoother this whole bike is than my previous derailleur bike was, although that is only in part because of the hub.
I'm sure if I had a bike that was made of ultra-light materials and stripped of all unneccesary components, I would notice a huge difference, but that's not how I ride. I tie on every piece of equipment I might need, add racks and fenders, and don't think twice about throwing something in the my bags "just in case." Because that's my attitude, the weight of the hub means nothing to me.
Is the hub overkill? I'm sure it is for many people. People who race think that a rack and fendears are overkill. People who ride fixies think any gearing at all is overkill. One guy in my area rides a unicycle to work. He probably thinks a frame that consists of more than a front fork is overkill. But it feels about right for me.
And if I were about to buy a 40+ lb. Cargo bike, I don't think I'd spend too much time worrying that the Nuvinci might be too heavy and built too sturdy for my bike. There are reasons not to go with a Nuvinci. The weight could be an issue on many other kinds of bikes (although I'm running the hub on a touring frame), but not a cargo bike, I wouldn't think. But the gear range is not as wide as a front and rear derailleur set up. The price is higher than many other drivetrain options, and only Rohloff makes a more expensive IGH, I think. Some people don't like IGH's in general because they are not as straightforward to maintain if something does go wrong with them. Those are all valid concerns, and I could see someone deciding against an IGH system because of them. But for a cargo bike, I think it's well within a reasonable weight range, and, on a cargo bike, or any bike, I don't understand the concern that it might be built to stand more than you can throw at it. I would put that in the "pro" column, not the "con."