For the question in your title. They are separate distributions because different teams within NVIDIA work on the different integrations. If you want a branch that has them all merged, check my signature.
Now for the other question. FLEX liquid works with the particle systems, just because thats what NVIDIA decided to use, particle systems are very efficient in rendering large amounts of particles. This does mean its not very good at placing bodies of water around. However I am working on a component (Had it working in my 4.9.2 branch, just porting it across to my 4.13 branch) that allows u to manually place bodies of liquid (particles) around, but has the same rendering efficiency as a particle system (as it uses the particle system renderer under the hood). You can see an example of it in action here: https://www.youtube.com/watch?v=4prOdjlSd-Y
It’ll be available in my 4.13 branch in a couple of days.