How likely a car is to fall off the jack, or any other support depends on a lot of things so there isn't a blanket answer. Use a three-ton trolley correctly and it'll be fine. Use a scissor jack on gravel and your car's probably going to end up on the ground.
incidentally, for the service I lifted one side of the car with the trolley jack using the turret shown. I had to remove the plastic trim to get at it. I then just jacked the other side on the sill and left it on the jack. The only really issue with these jack points is that they are very close to the sill support/jack points so jacking then getting an axle stand in is tricky.