Several people have commented that opacity seems to work odd for them.
On the left is b) basic 2 opacity, and on the right is b) basic 4 flow opacity. The only difference is that the right one has the flow curve enabled. It seems that the flow is cut off by the total opacity instead of the opacity of the stroke at that moment, making flow difficult to control.
I am making a Krita Abyss task for this because I suspect it is quite complicated thing to fix, but I would like to discuss it at the sprint.