When ATVs first came out they were three-wheelers! Talk about instability!
Still the center of gravity of a four-wheeler with a rider is quite high, so it doesn't take much side slope or up-slope to cause a big problem. Everyone should take an ATV safety course before taking one out in the wilds. Still, they're much more stable than a trail-bike, especially in slippery soils.
“God save us from people who mean well.”
― Vikram Seth