I'm not saying CW soldiers necessarily had it worse, but that I certainly think you're over generalizing and not really thinking this all through. The nature of the soldier's trauma might have been different, but it was there, and it often was daily. I just don't think it's right to assume either one was worse than the other.
Yes I am generalizing.
My main point is. From what I know about the topic. That is what I have read the last few years and what I know from being in the danish army (2006-08, but a knee injury in a soccermatch got in the way of my deployment to Afghanistan)
and following how we here in Denmark handle the problem - Then the killing is not main problem, and not something you can do anything about anyway. anxiety, stress and Lack of rest is.
So for a union infantry soldier (say from Maine)in the army of the Potomac in summer 1861- summer 64, he was not in combat for that many days. And for long periods (month) after a battle the armies pulled back to camp to resupplies, rest and refit. So this should help make the % of soldiers who get PTSD lower.
His home was not directly threatened. And yes there was the risk of disease, but people dying of disease or simply accidents was ore common back then, than today.
So I
think that the % of men with that experience who had problems would be smaller than the % from a US infantry units who served for combat tour in Iraq or in Vietnam for that matter.
But we can of cause find cases where that is not true. Serving in the overland campaign and siege at Petersburg with daily combat and a daily risk.. Having family in the combat area. Just being on the csa side might have an effect. Likely had a clear negative effect.
But the basic problem is that we don't really know much about it when we are talking the civil war. And even today there are much that is not known about how and why some people gets PTSD and others don't.
So as you properly noticed, I use words like think, believe, likely... since this is just my opinion, that I really can't back up at this time.
And even if only 5% of all civil war soldiers ended up with problems, that still a lot of men.
(to pull a more or less random number, that I just know is lower than the % from the US army today)