It is funny that there is this widely held notion that small cars are a death trap in an accident. It just isn't true. Insurance industry statistics for North America show you are more likely to get injured in an SUV or other type of small truck. These go back to the days of even the original Mini. I don't see anything to worry about.
The SUV stats reflect that the most common crash scenario is a rollover, which is understandable given the hulking, high center of gravity nature of the cars along with their general confusion about handling. Plus idiots frequently don't wear seatbelts. (oops should I not say idiots
? Sorry if I offended anyone!)
Small cars don't tend to roll, but the biggest problem there is physics. If you in your MINI get plowed into by a some brobdingnagian SUV, the difference in mass is going to ensure that you get the worst of the deal. F = MA, amiright?
The advances in structure and airbags help mitigate the disparity in mass. So people are surviving crashes that would have killed them thirty years ago, even when some pinhead launches their Hummer at your windshield. Said pinhead, however, is much more likely to flip his monster on the way to get groceries, and coupled with the feeling of invincibility that a sadly large number of these people seem to have, the result is flattened pinheads all over the place after they roll their hulking beasts.
Here in AZ you almost can't drive for a month without seeing some ginormous SUV upside down in the intersection following a crash. It seems routine, and the words "rollover accident" are pretty much a staple of the traffic reports. Very few of those are small cars (though they do get rolled when crashed into sometimes).
The MINI, despite the rating, seems a strong little car and the occupant has the benefits of a lot of airbags. It's not likely to roll over, and even if it does, rollovers don't seem to be that damaging in a MINI. If Pinhead careens out of control and plows his SUV into you, well, with luck the airbags will save you.
It's up to you to increase your chances. Never be the first person off a green light at an intersection, because Senor Pinhead is going to be blasting through trying to beat the red. I look both ways before I enter the intersection, and if there are cars approaching the intersection in cross traffic, I tried to keep sacrificial cars between me and them, just in case.
I assume that everyone on the road is trying to kill me. I assume the worst on the part of everyone around me. I never, ever tailgate, and when some fool is riding my tail I increase the following distance to 5 seconds or more so I have time to stop without him crashing into me.
At lights I leave enough room to escape if a random chowderhead decides to lock it up and try to crash into me. I watch my rearview mirror constantly at lights until I have a buffer of 3 or so cars to absorb the impact of Senor Pinhead in his SUV.
A paranoid mindset and defensive driving can go a long way to make up for a marginal crashworthiness rating.