Just don’t build them in the path of a tornado. :-)
Well, I live in tornado alley (St. Louis) and trailers don't seem to get hit any more often than other dwellings. They may (or may not) be more dangerous when they do get hit, but for the money they are a good deal. I live in a low-income area and I have considered renting one and decided against it - but not because of tornados.
Trailer parks are called “Tornado Magnets” in this area.