Flex is particle based, Flow is voxel based. By its nature, Flow is better for things like smoke/fire because of it, while Flex is better for something like water. I think Flameworks was replaced with Flow, same as how GI Works was replaced with VXGI.
I’m not sure why Turbulence was added, since most of its functionality is a part of GPU particles as a native engine . It’s always good to have options though I guess.