The fluid will flow through the path of least resistance. On a single top down rad, the fluid enters the top and is pulled out the bottom, if the inlet and outlet is on the same side, the fluid closest to the in and out should move faster through the rad, the tubes farther from the in and out will move slower and remove less heat.
Top down single with inlet and outlet on opposite sides will have a more uniform flow.
Cross flows are still more uniform, and a dual or tripple cross flow even more.
