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Good balance of stiffness to your weight, power of your turns, your board surface areas and rails, and wave speed is best.
Too stiff, feels stiff and won’t initiate easily, can spin out when overloaded.
Too soft, feels mushy but easy to start turns, but ending turn are vague, and you don’t know where your board is going by feel.
Stiff is also bad for weak finboxes or light glass jobs.
Soft is horrible for surfers who are powerful and pinpoint.
If a fin is not stiff, then the flex will have some impact on performance, with the fin bending in cant and in toe-in when loaded. Most of the time if this is an unintended consequence then it makes the fin feel mushy. However, there is an entire subset of fin making for which flex is desirable and intended, and the flex can aid in performance.