Most unibodies are designed to bend and flex, to absorb the stress. So to some that are accostumed to stiff frame bodies, they get the impression its weak. Nope, its getting the job done by avoiding cracking and deforming with the give of the bending and flexing, and its a lot lighter and cheaper to build. A simple frame starts to bend and flex it will crack or deform, so they have make it heavy and strong. A unibody can bend and flex, so it doesn't have to be real heavy to avoid the cracking and deforming.
You can make unibodies very ridigit, but thats either takes more metal and weight OR more time designing it and complexity building it. Which more and more desginers are doing today. And many Unibodies have excellent and ridigit chassis that are great. Which most people prefer, its not re-assuring to have the car bending and flexing, even if you know its suppossed to do that.
I tend to "think" the XJ went with the heavier design, the frame rails run the full length of the body, most unibodies don't do that. My '69 Charger doesn't have full length frame rails. As well, several of the crossmembers and the rails themselves are pretty thick steel.
My Neon R/T on the other hand, is pretty thin metal, but its got fairly complex shapes to the metal and connection points and frames and crossmembers are pretty intricate and complex, there is even a heavy steel tube welded across the center under the rear seat tieing everything together almost like roll cages do sometimes. Say what you want about Neons, but they do tear up the auto-cross circuit in the light-weight class, because of the ridigit chassis that gives such great handling and raw cornering with some mods. There only problem is they occassionally flip over in atuo-cross, because if they get into a heavy lateral motion, like a slide, and then suspension on the outside digs in and bottoms out, the chassis is so ridigit it doesn't give and flex, it will flip right over.
