Is Rolling Difficult To Program?

Exactly, it is about right conditions, you need something that upsets steady to make it happen, but it does happen quite easily when there is something that upsets stable state when you have everything right.
For example FWD cars tend to get rollovers from kerbs, when hit right way, this is Type-R car with quite low CG to my knowledge:



Renault Clio cup race cars seem to do that too, just a little something that bumps right front upwards made that Type-R go over when cornering hard.

Incar of rally car rolling, here there was a ditch next to road which flipped the car, but from experience I can tell that car is easy to roll on grass field, but you need some roll to begin with, tires need to dig in so that it has to land sideways while having a roll already and roll happens easily.
https://www.liveleak.com/view?i=a3e_1461702539

Steady turning or sliding to side is no problem, but when there is roll involved, like going from left to right repeatedly, or at start of turning, landing sideways etc. those make rolling easy.

For example here, in blurry mess of pixels, car goes sideways, grass field is lower than the road and that drop starts roll, that drop makes wheels dig in and result is a rollover:


Grass rarely is same way on level with road like in game, also grass fields do have a depth and soil is kinda sticky if you manage to dig into it, so rollovers don't happen so easily in games, but in right conditions it does happen quite easily, it is not hard at all, those conditions just does not always occur, but when you learn what variables affect it, it becomes quite easy to see potential for such event.

Overall I don't find grass quite satisfactory in games as it is usually hard flat smooth surface, I recommend people to drive 50kph on grass that is side of road etc. to see how flat it is, with truck you don't drive on grass, depending from moisture percentage of the soil, it can sink deep or really deep and thing will get stuck.

Wet soil and not compacted too hard, that is something you don't drive off even with a car, even slightest incline tends to be too much. Grass varies a lot, hugely a lot depending from moisture and how compacted soil is, places where is not driven can make really bad place to drive into.

I'm not sure if it is possible to get groundmodel set so that you will have ok grip when kinda floating on surface, but add more weight or spin wheels and you can kinda dig into stuff, it kinda would need to increase friction/resistance deeper you sink to be anything like real soil under grass.

 

Christ sake, I need a notepad, this just gave me an idea; Long stretch of road looking kind of like that, with groundmodelled grass on the side of the road.....anyway, that's all pretty cool.

 

I'm gonna start working with the track editor a bit so we can have a couple sections of long, wide road with different friction levels so we can test.
This will take a while however as i'm on holiday.