Actually, while the Sonic might be listed as a "unibody," it uses full-length, high-strength frame rails. A true lightweight unibody design that I'm talking about doesn't have full "frame rails" as such (look underneath a Honda Fit and you'll see what I mean). They're molded so the body eliminates full frame rails to save weight. When I had a 1995 Honda del Sol, I wanted to get a hitch for my bicycle and was told that (at the time), there wasn't any way to mount one because my car didn't have frame rails. He showed me underneath on what he was talking about. Today they can mount a hitch on the Fit (and del Sol), but one I saw they had to have bolts go through the spare tire compartment in the body and that's not an ideal place to put stress from a trailer tongue.

As for warranty, my car has been out of warranty due to age and miles so I'm not worried about that.