I was an IV RN for almost a year, and did multiple IVs whenever I worked (yes, I did a lot of other things, too, but when I was working I was designated the person to call for hard sticks or I'm-too-busy-to-do-it RNs). I became really good at putting in IVs.
When I changed departments and was no longer in that position, within 2 months my IV skills went way down.
There were many, many nurses who only did 3-4 a month, and had no confidence in their IV skills. It's not something that you can just watch a youtube video and suddenly you're an expert. It's not hard, but it does take a lot of practice to be competent. If you're in SHTF situation, how many of your patients will realistically be early 20s and healthy?
Elderly often have small, weak veins. Prednisone skin. Diabetic or kidney problems means hard veins, some of them will feel great but you won't be able to get any blood out of them (they're "dead" veins). Drug abuse, dehydration, long term illness, long term medications, low blood pressure, these are all things that will make an IV start much harder, and yes even experienced RNs have problems with them.
Yes, some are very easy. Some are not. But remember, military personnel don't have long term medical issues. They are all relatively young and healthy.
I've done a LOT of IVs, and it is honestly not something I would recommend without training and practice.
As has been mentioned, a lot of things can go wrong. Infiltration, infection, hit an artery. If you put an IV in the leg of a diabetic, they can lose their leg. Give fluids to a person with a history of CHF, not good.