Draw a classical sector. When you shift-drag the handle it will snap to the next angle as set in Tools > Options > Draw > Grid. Do the same with a custom shape sector and you will notice, that it does not snap.

Draw a classical rectangle. Set a vertical snap line over the left third of the rectangle. Drag the handle for the corner radius. Notice, that it snaps to the snap line. Do the same with a custom shape rounded rectangle. The handle does not snap to the line.

Feature request: Make custom shape handles snap to snap lines and make polar handles react on angle steps, same as the handles of classical shapes do. Such feature is needed, because there is no UI to set the exact position (or angle respectively) for custom shape handles.