When I connect two rectangle nodes with an arc edge or a bezier edge, the curve of the edge is not smooth. I can clearly see discrete line segments. (This is especially pronounced when I zoom.) If I export the diagram to PDF and view it in Preview on macOS Catalina, the edge is similarly not smooth.
Here's an example screenshot from yEd showing this:
If I do the same thing in OmniGraffle, the edge is very smooth, and it is similarly very smooth in the exported PDF when viewed in Preview.