We’ve put two bolt in cages into rally cars that take part in the Targa Rally. Both cars, 2 years apart, crashed and both were a right off. Only reason the drivers and co driver survived was because of the cage.
If its designed and built properly then their is nothing dangerous about it.
My assumption in this case the dude with the Mustang simply drilled the holes through the floor and bolted in his cage instead of welding solid plates to the underside of the floor and then sandwiching the Cage inbetween etc… so the 18-20ga metal the floor is made out of ripped a part.