I suspect eel heavy lists will have a tough go of slaanesh, though that can probably be said of a lot of elite armies that will be facing the new hedonite book. You're looking at an army that's nearly as fast, can potentially ruin high tide for units that need it most, and whose army special rules and across the board rend are well tailored to eel hunting, morrsarr especially. That said the army is fragile, and will rely on perfect positioning to a higher degree then some of the more forgiving factions.