You seem to assume that the "other" system would have made everything faster and higher throughput. That was not the plan, at some point the devs even thought about adding viscosity to the different fluids which could in turn result in slower fluid transportation speed. You know, heavy oil, crude oil, I would think that should move slower than water. So be careful what you wish for. In the end it might have been more heavy on CPU calculation and you still would need more pipes.
You just don't have infinite throughput with pipes. Same thing with belts, it's just not a number written in a tool tip and nobody is complaining there.
Also, about predictability: If you build a pipe in the real world and attach a junction, do you really think a fluid going through that pipe would split perfectly evenly? No, it would not. Never. So why should it do that in the game? This has nothing to do with how the game calculates fluids, but why complain if it doesn't even exist in the first place?
I personally am glad how the fluid system turned out to be. I take better optimization over some edge case predictability every day.