Free-ranging snakes representing dozens of species from around the world are discovered in the United States in any given year, usually as a result of escapees or releases from the pet trade, but most of these don't appear to have established a reproductive population.
Any animal can be problematic when released in places where it is not native. For example, the brown tree snake was introduced to the American island of Guam shortly after World War II; it has decimated the native birds, mammals and lizards of Guam, such that only a few small species remain. Guam has now lost 10 of its 12 native forest birds, most of its bats, and about half of its native lizards. Burmese pythons introduction to Florida have been linked to severe mammal declines in Everglades National Park.
None of these snakes pose more than minimal risk to human safety. Human fatalities from non-venomous snakes in the wild are very rare, probably only a few per year worldwide. All known fatalities in the United States are from captive snakes. Predatory attacks by invasive pythons are a possibility, though the rarity of such attacks suggests that they are highly unlikely.
Learn more: USGS Invasive Animals