I was looking how the new Sankey Diagrams work in 18.2, and I liked this feature a lot. What leaves me perplexed is the vertical arrangement of the milestones. Some of the milestones are placed above the complementary flows, and there is no obvious way to avoid it.
In a basic barbershop-style model, I got this Sankey layout:
Please note that the milestones "MS2B - Customer Leaves" and "MS3B - Customer Returns" are placed over the complementary flow from "MS1 - New Customer" to "MS2 - Start Waiting". This placement of the milestones doesn't help to evidentiate the alternative flow in the model. It also looks like there is a flow sequence MS3B-MS2-MS3-MS4B, which is not possible in the model.
In this case MS2B and MS3B should be placed below the flow line from MS1 to MS2, and the diagram should avoid intersections of the flow lines with unrelated milestones.
The diagram would also look more orderly if the top edges of the major flows were aligned.