It can be, especially with minor traumatic injury cases. Often times the various types of imaging that may be performed (this typically includes MRI and CT scans) will show no objective abnormalities.
Traumatic brain injuries are often only able to be diagnosed by a qualified physician who regularly treats minor TBI patients (such as a neurologist, for example) and by a neuropsychologist who is trained to determine the loss and/or impairment of brain function caused by the injury itself.
Because these types of claims can be extremely difficult to prove, hiring with a qualified personal injury attorney familiar with TBIs and MTBIs is important. An attorney must understand the legal, as well as the medical components, of a brain injury to represent you best.