While a waterfall appears like a nice, solid background they actually have a lot of holes in them.
A nice, sharp, frozen-in-time photo of a waterfall will have a lot of detail. Detail that is distracting from your original intent to take photos of whatever is covering up that interesting detail. A silky smooth waterfall is easier to get and better to give the impression of falling water. The portraits will pop.
Using flash and long exposure gives you silky smooth with some sparkle and a bit of the waterfall violence mixed into the image. Watch out for the peopl's shadow on the waterfall. It looks very bad. Don't ask how I know.