If the lower strap wasn't a bungie it wouldn't be the worst ever in theory. Knotted 5.5mm Spectra is rated at 14 kN, which is about 3,100 lbf. You've got problems if your passenger compartment mounted 40 pound (18kg) Hi-Lift gets to 14 kN. That's a deceleration on the order of 65 MPH to zero in .05 seconds. Mind you not the car, but the cargo. You'd have to hit an absolutely immovable object. A bungie, yeah, that's gonna break going over a speed bump at Safeway.
That said, if there aren't two 3/8" bolts holding down my Hi-Lift, I don't feel comfortable.
I've used my Hi-Lift a few times. One time as a jack during a field leaf spring re-weld. Another, before I had a winch, I had to recover myself 4 feet at a time out of a snow-filled ditch after skiing up at Butler Gulch.