Every diabetic is different, but I was raised by my mother who is 85 now and was diagnosed with diabetes and has been on insulin since she was 10 years old. It is possible to go into a coma and die in your sleep from a low. My mother has been as low as 25, and as a child (my dad worked nights) my siblings and I had to get sugar or orange juice for her at least once or twice a week. There were times that her sugars were so low that she did not know who we were and we had to force sugar water down her.
Her doctors have told us that it is more likely that she will die of a heart attack or stroke brought on by the low blood sugars and have led us to believe that she could die very quickly of a low blood sugar.
Low blood sugars can cause loss of brain cells, but not near the damage that high blood sugars do.
As I said, my mom is 85 and while she forgets a lot of things, she writes checks for her own bills and takes care of her own bank accounts accurately, so she hasn't lost too many brain cells.
My mother has never had a lot of highs, because she was raised during a time when she could not have sweets, sugar etc, and to this day will not eat anything that is sweet or has added sugar to it. She has not suffered vision loss, amputation, kidney failure or other problems associated with uncontrolled or poorly controlled diabetes. She has some mild neuropathy in her feet, so she has to use a walker for that reason.
I am not a medical expert on diabetes, but am going strictly from my personal experiences growing up and dealing with my mother now.