After doing some experimenting I understand a bit better how fluid flow works in factorio, fluid rates depend on the gradient of fluid between adjacent pipes, so long pipes lose troughput because the gradient must be smaller.

That being said, the ducts compare kind of poorly to regular pipes, I think the reason is that the troughput/gradient should be higher to really get 4 times the troughput (probably 4 times higher, but I haven't worked out the math :-)

I'll link an image with the results from my experimenting:
https://imgur.com/a/HCwFxLI
To summarize, the ducts have no benefit when used normally, but are actually ridiculously powerful when used in some ways
Hope this will be fixed, because I really like the idea of the mod, right now I'm using the system with the non-returns which in my opinion is a bit goofy and overpowered, I'm not sure if it was intended to work that way

Welcome to the magic world of Factorio fluid mechanic!
All work as intented, in fact, if u try to do the same (the bottom test) using only chained vanilla pumps (no pipes or less as possible), u will get a strange output too (if the vanilla tooltip is right the max vanilla pump throughput is 12'000/s!!! Remember that pumps have 100 unit of fluid for 4 level of height, they are pratically a compressed ducts).

The true meaning behind the ducts, is that ducts can grant a nice flow, without play with an evident cheaty behaviour, like placing pumps everywhere only because the base engine is impossibile to use with larger setup and because instead use 1 unit of fluids in vanilla they are used 10-100 units for time...

Anyway, I will never rebalance this, they work perfectly. An expert Factorio player can create nice vanilla pumps labyrinths obtaining the average ducts throughput, forced to use pumps with a tedious patterns, instead a less expert Factorio player can use this mod and play with "creative" setups gaining the wanted flow without have to study the pump cult.