IME, there are is no down side to Sargassum. Control by hand harvest is easy. When its growing, it can grow fast, and tends to grow up from single holdfast rather than spreading laterally across the rock. I doesn't spread easily by fragmentation, like some other macros, either. Since the fronds are bouyant, you can let it grow very tall in the tank, even up to the surface of the tank. If you have a veg filter or refugium, you can thow fronds in there and let them float, and they will continue to grow.
Over time, growth may slow down and eventually stop. Enjoy it while you have it, its one of my favorite algae.