You're right, it's not that easy. I worked in the VA system for three years during residency, and it was relatively rare to see a veteran who was considered 100% service connected. I had a patient who was exposed to Agent Orange and later developed Hodgkin lymphoma, and while the cancer was treated successfully, the surgery and radiation left him with permanent disabilities that impaired his day to day life. Even he was not determined to be 100% service connected. (He came close - something like 80% if I remember correctly - but not 100%.)

I personally never saw someone who was determined to be 100% service connected purely for PTSD, but I am also not a psychiatrist so it's possible that there is a selection bias there.