Yes and no. I agree that concusion can be difficult, but brain injury, such as SSG Bales has been described as having, might actually be easier to diagnose. X-rays and MRIs can tell a doctor a great deal about an injury.
To the best of my knowledge, the type and extent of his brain injury has not been revealed. Nonetheless, back in "the olden days", had someone been diagnosed with a brain injury, it would have meant either a medical discharge or the individual would not have been deployed. Today, with a sharply reduced military that has been deployed again and again and again, there simply isn't the manpower available to either medically discharge Bales or keep him home.
All things being equal, it remains, in my mind, that the practices of the "olden days" should still be used today. Even if Bales had ben allowed to remain in the Army but not deploy, he would not be charged with 17 counts of murder and 17 Afghani civilians would still be alive.
The nature of a brain injury, as you noted in your post, is that the individual becomes unpredictable in the best of circumstances. Sending that same individual back to a stresful, presure-cooker environment such as a battle zone is is nothing short of madness.
Brain injuries, even with X-Ray and MRI, are still terribly hard to diagnose, if for no other reason than individual variation between brains can be enormous.
I saw my first real jaw dropping example of this in an old NOVA episode, of a girl with water on the brain as a fetus, who eventually had a shunt inserted in her neck to balance spinal fluid pressure. But by that time her brain had been compacted to about 1 cm of tissue on the inside of her skull. All the rest of her brain area was just fluid.
At the time the episode was filmed, she was graduating from high school, with the only indication she had almost no brain being a slight limp, limited use of one arm, and a small speech impediment. Astounding.
Two different soldiers may have almost identical injuries, yet one is ruined and the other just shrugs it off. A neurologist can initially only diagnose by “capability and impediment” in physical movement and cognition, how the soldier feels subjectively, intellectual capacity and memory, and things like that.
Definitely overlapping with PTSD and other mental and physical conditions.
This is why neurologists get the big bucks.
Deciding who stays on the battlefield and who goes home must be incredibly difficult. Based on what his peers said, this SSG was totally asymptomatic before going on his rampage.
Only now will he possibly get the medical microscope he needs. He cannot walk the streets until they either find something and fix it, or determine it can’t be fixed.