Why would it be dangerous? In most states, they are required to be inspected and licensed with the appropriate cooking facilities. The food is cooked to order on the spot- the biggest danger from food is from food that sit around. If there were a problem with food trucks, there wouldn't be so many of them, and they wouldn't be so popular.
The Roach Coach as it's affectionately known on the construction sites is probably one of the safest foods that you'll ever eat. The food is mass prepared by a certified food preparation company the truck owner has to be insured. And the food that served is kept hot.
There are of course companies operating outside of the law. I worked at a tomato packing house for 7 years and we had a roach coach that came by every day. Nobody ever got sick