To use a jack on a unibody, there are pinch weld adapters and pads available. Certainly a lot safer than using some of the factory jacks that come with some cars these days. Some folks have also been known to use a hockey puck with a groove cut in it as a cheap pinch weld adapter with their floor jack.
It's not that, It won't roll under the car

Ok, duh... I get what you are saying, but if it's a unibody car that you are working on then usually there is not really a good place to jack from underneath, so no reason to put a floor jack under there at all.