The Mini is much like most other '60's classic cars in terms of crash-worthiness. They are not and never will be as safe and crash-worthy as a modern car.
However, if a person is into classic cars that won't deter them and they will restore them to be as strong as they were originally and they will drive them defensively.
If someone has a serious concern about driving in a classic car, then it is best to stick with modern cars really.
In addition to Minis, my Grandson & I are currently restoring an MGB-GT and that has the same safety issues as the Minis, but we love classic cars.