The Nihola can turn. If we just concentrate on slow-speed manuevers for a moment, it is very well behaved. I can't imagine tipping over at low speeds, on level ground.
The turning radius is kinda big, almost car-like (and worse for some of the extra-size Nihola models), but if turning around that way is too much work, you can get off and lift the back very easily, and pivot the whole thing on the front wheels. Turning radius of zero. This works extremely well due to the position of the front tires, which are positioned to take most of the weight of the trike and around 100% of the weight in the box, so it works great even with two kids and a pile of crap up front. Just jump off and pivot 180 degrees, jump on and go again. Point the wheels straight while doing this. Also having a rear cargo rack is nearly essential, to provide a handle. It also works badly when you put a heavy load on the cargo rack.
Turning on a side slope is less easy, sometimes. Basically if the hill is too steep you can't balance on it. Also note that its easier to balance when the nose points down the hill than when it points up, because when it points up, your weight is balancing over the one rear wheel. The trickiest part of a turn on a side-slope is the part where you are pointing uphill, maybe 45 degrees offset from directly up slope. Its an interesting thing that its actually easier to go straight up a hill than angled up a hill. So all that said, it has to be a significant slope to pose a problem. On such a slope, it might be easiest to get off and turn by lifting the back.
Turning by lifting the back is a really awesome feature, in my opinion.
A trike such as the Nihola needs to be slowed for corners. There are a few places on my commute that I slow down much more than the bikes need to. Still I suspect that it can be cornered faster than any other cargo trike, because lifting the inside wheel is not really a cause for much concern. It just means you have reached your cornering limit, and should fix the situation before you actually do tip over. Basically, you are driving an oddly balanced bike at that point.
I sometimes deliberately run the inside wheel over a curb or dirt pile or whatever so that I can straighten the curve out and conserve momentum. If I take the corner correctly, the inside wheel has so little weight on it that this happens smoothly. While often ridiculous, this is actually a very sensible strategy at a particular place where I enter a road from a sidewalk with a small ramp, and turn sharp right. I can drive the inside wheel right off the curb, while the outside wheel drives down the ramp, and dramatically reduce the amount of left-right steering and associated slowdown.
So, what about the situation where a Nihola rider finds him/herself going too fast when a turn needs to be made? Well obviously scrub as much speed as possible before the turn starts, squeeze especially hard on the front brakes because the back can't contribute much (unless you have heavy pannier bags or something back there), and might start to slide, which is an unwanted distraction at the least. Also, hard braking and hard cornering at the same time will require that you lean a lot and hold on tight. Braking while lifting a tire sounds like a bad idea to me.
A Nihola will reliably understeer by lifting the inside wheel when asked to turn too sharply, but on some surfaces can be convinced to oversteer somewhat by using the back brake, together with the front brakes, a big steering input and considerable speed. It is most easy to oversteer while simultaneously going down a hill, braking, and turning on a wet road surface. Oversteer can quickly be corrected by straightening out the wheels and releasing the back brake. I have never managed to do a 180, even in the snow, although I have certainly tried. Probably just a matter of finding a suitable sheet of ice.
Some day I'll make a post about driving a Nihola in the winter, where I can go on about handling at the limits of adhesion.