Stability
Too much of Rudder plane surface leads to Directional Stability.
We must appreciate that improving stability in one aspect invariably leads to compromise on some other aspect.
Too much of Lateral Stability leads to dutch roll as in a bank the aircraft wants to come back to its neutral point.
Too much of Directional Stability leads to spiral instability.
As a rudder is an aileron in itself, it Will suffer Stall (seperation of airflow) just as a wing would. So if you exceed the rudder's crical angle of attack, your rudder Will stall.
The stall can be delayed by adding dorsal and ventral fins to it.