The caution is not so much about gambusia, but any introduced species. No matter how innocent a fish (or any creature)may seem, when you introduce it into an ecosystem that has evolved no checks and balances, the outcome is almost never good.
In much of the Southern US there isn't much choice, because you will have gams in your pond regardless. Fine with me, becasuse all my fish find them delicious. If there are no gams in your area, it's probably best to leave it that way.
