a big chunk of the us problem has been from domestic hogs going feral and breeding. Hogs are an extremely adaptable species and a domestic hog will go ferel in a few weeks with a thicker coat, eleongation of the snout and rapid tusk growth. combine this with a high degree of intelligance, rapid breeding, large amounts of aggression and a highly adaptable diet and you have a strong survivor. Having grown up on a hog farm, well, anyone who underestimates hogs, especially large ones, is likely to get very hurt very fast.
I know they have been a major problem in southern US. pretty much anywhere they can get into large tracts of unregulated land, they will thrive. As far as eating goes, I don't think a lot of people do now, although wild boar used to be a strong staple of many diets.