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.
People are like rifles. Some are tried and true, having great eyes, personality, and fun to be with. Others never seem to hit the mark with you. Still others go off half-cocked. Still, it's nice to know most of them.