So 99% of the time it's a guaranteed instant lights out. Freak incidents happen. Might damage only the frontal lobe, might not have placed enough pressure on the brain stem. The brain stem controls respiration and heartbeat. Sometimes that 1 in million shot happens. Ever seen the video about the chicken that was decapitated, yet live for quite some time. Weeks... The clever removed everything except brainstem. Essentially a walking zucchini, but, the chicken for whatever it knew, was gone.
Ask yourself. Would you rather just out of nowhere just cease to exist, no pain, no fear, no cognisance at all or what just transpired, or take a hit to the heart/ lungs/ liver... Which you will very much feel, and possibly be conscious long enough to gather what has Happened? Unless you've severed the artery feeding the brain and supplied ample hydrostatic effect, there is a larger than nil chance that enough Oxygenated blood will reach the brain to allow cognizant experience for although brief, still painful and terrifying moments. Even though there was breath and pulse, there was brain death.. it had no idea.