Some use alcohol, which reduces the corrosion problem, but it doesn't weigh as much. Tubeless tires should be filled above the rim or ran dry, otherwise corrosion will get you. Generally fluid means tubes. Radials won't work right at low pressures with fluid because you loose all of your volume, which is what allows them to work to begin with. I think fluid will limit you to 16psi minimum, which kinda defeats the purpose of radials to begin with.
Cast iron is by far the preffered ballast.
Fluid has too many problems when it's only real benefit is that it is cheap. I am convinced that the right setup for most use is enough tubeless radial rubber to carry the necessary load at 6 psi, then carry as much properly distributed iron as possible without having to increase pressure on the most limiting axle. I'd leave the fluid for the motorgraders and wheel loaders.