Even the stoutest of ramps have one fatal flaw:

If you're pulling on a bolt at 90* to the length of the car (say, a torque converter bolt) the ramps can and DO tip sideways. Both the guys I know who got killed (one I knew personally - he was a HS buddy) were killed in this way. His Dad was an engineer at the GM Proving Grounds and made the ramps in question out of structural steel. These ramps were STOUT. Granted it takes a pretty good pull to do this but when you're under a car trying to accomplish a task the tendency is to yank as hard as you need to and you may not notice that the car is a bit tippy.

If it was up to me I'd make these illegal to manufacture or sell in the US. I have a hoist so this issue goes away for me. If I have to work on the floor, out comes the floor jack and stands.

All this being said, you can probably do an oil change and other low-effort jobs with the car on ramps and on a solid floor and not experience an issue. But there's a tendency to think that you have the technology to raise the car and the next thing you know you're trying to pull the trans or some such and that's where the danger starts creeping.

Of course, another issue is that of convenience - the ramps SEEM more convenient but if you have to do anything with a wheel in the air (say, brake of suspension work) the ramps are a PITA.

Dan