I've owned the 5000SLX QuickJack 110VAC kit for 8+ years.
I've worked underneath the car to do various jobs - upgrade exhausts, swap-out rear diffs, replace/install suspension components, change driveline fluids, etc. The lifting platforms are stable, and I ensure the lock arms are against their stops before going underneath the vehicle. I have jackstands that get placed under the vehicle for "just-in-case" failures, but if you take the time to set up the lifting platforms, ensuring they are squared-up and centered under the pinch weld lifting points (all my vehicles are uni-body), then even at the highest lift point, the vehicle is stable.
For instance, my latest use of QJs - installing a Borla exhaust on my Audi SQ5 Sportback:
One key lesson I learned, a.k.a. "Pro tip" - after all other connections are made, before connecting the last quick-disconnect (QD) to the Powerpack, plug in the Powerpack, depress "Down" to allow the pressure in the line to bleed-off to the reservoir, and then the QD will easy attach to the Powerpack fitting. Otherwise, you are fighting hydraulic pressure in the Powerpack supply lines while making the connection.