Post car hydraulic lifts have 2 posts and 4 arms with their "palms" (contact points) being flat.
See example https://postimg.cc/vgtrrQc3 taken from a random youtube video.
The main concern is that the palms are flat. There appear to be no mechanical restraints other than the friction between the palms and the vehicle's frame where they touch each other. When a car is not ideally aligned respective to the center of gravity and with enough force pushing the car in one direction, I assume the car may slide off the palms and hit the ground. What force is required to push a raised vehicle off the hydraulic lift arms? Imagine, two die-hard people push the raised car (just for science). Can they flip it over?
Compare with jack stands that usually have a curved "pincer" that restricts the horizontal movement of a vehicle's frame bar.
How would you assure a person who needs to work under their car that the hydraulic lifts (when the car is lifted by a professional auto mechanic) are safe?
As MountainManJoe says, they're fine as long as used properly. Hydraulic lifts are used constantly in shops all over the world. It's almost invariably carelessness that leads to incidents like the following...
https://www.youtube.com/watch?v=nhFWoyq8OGA
They are completely safe if used properly. Shops wouldn't use them if they weren't
