What to expect just depends on the individuals there at that particular hospital.
I think the admonition from your uncle is derived from veterans that, due to PTSD or any number of other things, just never really readjusted to civilization back home. There are a number of vets from Vietnam and every war since that find extreme difficulty in returning to their lives.
After a while, this leads, for some people, to a very debilitating way of life and a state of mind that victimizes itself and lashes out at things and people openly.
So you may have been warned about a type of attitude you may or may not encounter (or be inundated by) when you go.
As someone who has mild PTSD from deployment, I can say that, given the right situation and level of trust, it helps me to talk about what I've seen and done. Other people just aren't ready for that yet, though, so... it just varies.
I support your decision and encourage you to give it a shot. You may also want to ask your uncle to clarify what he means, too - may give you some better insight on specific information.