It depends on the extent of the head injury. With a severe closed injury, you usually have increasing ICP (intracranial pressures) which can damage brain tissues and cause herniation if not treated quickly enough. Severe CHI usually include intracranial hemmorhage (there are different types - subrual, subarrachnoid, epidural, etc.). Those type of CHI can be a surgical emergency.
Open head injuries can be just as severe/deadly. But it depends on the type of injury -- with OHI, you usually have a penetrating or perforating type of injury. If the object passes through the brain and hits a major blood vessel, it can be just as severe as a CHI. The skull provides great protection, thus if an object hits the head and causes and open head injury, it usually does not penetrate deep into the brain because of the skull.
I feel as if I am rambling here.. lol.. but usually a CHI as far worse results than an OHI, but an OHI (gunshot wound or something similar) can be just as severe or more severe than a CHI. Thankfully our skull protects us from objects penetrating too deeply into the brain.