One major problem with it is that even in those dialects, a double negative can also be a positive, and outside of some semi-fixed constructs that mandate one o

One major problem with it is that even in those dialects, a double negative can also be a positive, and outside of some semi-fixed constructs that mandate one or the other interpretation in that dialect, the distinction relies things like tone and/or stress/emphasis, which of course are lost in writing. This makes it rather unwise to use in writing, unless context clearly dictates one interpretation.