The difference from petroleum-eating bacteria is that the genus found in these sargassum patches, Vibrio, includes some much more virulent pathogens. These incl

The difference from petroleum-eating bacteria is that the genus found in these sargassum patches, Vibrio, includes some much more virulent pathogens. These include (1) flesh-eating bacteria, (2) the species which causes cholera, and (3) the species that causes vibriosis, the intestinal illness commonly contracted from undercooked seafood or exposure to seawater. The common factor between cholera and vibriosis is the effect these bacteria have on the human gut, increasing permeability ("leaky gut syndrome"). As such, the Vibrio genus is "the dominant cause of death in humans from the marine environment" (quote from the article's FAU source.) On top of that, sargassum has been increasing due to factors like increasing water temperatures, and large amounts of it end up on beaches or just offshore. It is collected by people in various ways, and even sometimes turned into animal feed. That's not the case with deployments of petroleum-eating bacteria. Sargassum also acts as an important habitat for many kinds of marine life, increasing the number of vectors by which new Vibrio bacteria could come in contact with humans. This all means that the potential for a bad outcome is relatively high, but as for any actual concrete risk currently, as you say that hasn't yet been established.