When I was a medical student in 1972-76, there were still whole hospitals full of psychiatric casualties of WW II.
I absolutely agree that most veterans made successful adjustments, one way or another, to civilian life. Those who were in sustained heavy combat less so.
Severe disability from PTSD is not common, but it’s not really rare, either. It certainly wasn’t common if you, to borrow Patton’s famous phrase, spent the war “shoveling shit in Louisiana”, and since 1945 we have not had many men in heavy combat for 50, 60, and 70 days without a break.
Combat doesn't actually work that way; infantry units in any battle wear out very quickly and take casualties at a high rate, generally one dead out of four hit. During WWII in Europe, combat units were moved from frontline to reserve continually to avoid too much fatigue and too many casualties. Units that were left in place too long lost at such high rates that they were constantly replacing experienced soldiers with inexperienced soldiers and units had very few people at the end of a long stretch that had started at the beginning.
In battles like Tarawa, or Iwo Jima, or Peleliu, units were so attrited that they became combat ineffective long before the battles were over.
In Vietnam, I found the average time in combat before you became wounded or killed was a month and a half. Most of the infantry Marines I knew had been wounded at least once, some more than that and were still there hanging in there because they didn't want to let their fellow Marines down. (our grenades were responsible for a lot of our smaller wounds: there was no way to throw them far enough to avoid being hit by our own fragments. Many Marines didn't even report those wounds and self-treated them).
The phenomenon of lesser or more severe PTSD had more to do with the individual's nature than the severity of combat - but nobody can ever equate infantry combat with anything else.
That's one of the reasons why I consider the idea of young ladies in infantry combat as the height of idiocy.