They are actually amazingly safe for their size. I hate the things, but not due to their safety record.
Top Gear covered this (back when it was the real Top Gear, before the BBC ruined it) by crash testing a smart car into a concrete barrier. The results indicated that the car did amazingly well retaining the integrity of the passenger cabin. This is because they are designed to be extremely rigid and don't crumple. However, those crumple zones that are designed into larger vehicles are there for a reason: to absorb the energy of a crash, rather than transferring that energy to the occupants. Smart cars don't have that critical safety feature because they have no space to permit crumpling.
The conclusion was that while the car's structure will survive a crash pretty well, all of the force will be transferred to the occupants, and they will most assuredly be critically injured or killed as their brains smack into the front of their skulls, and other internal organs do likewise.
So, they're essentially "5-star crash rated" death traps.