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Like most things, Kung Fu has its strengths and weaknesses.
If Kung Fu here is defined as having the ability to fight well and occasionally defy physics, then Kung Fu's strength is that it makes one more self-reliant (against baddies) and more fit (better mind, body, and soul).
One of its weakness is the same as the weakness of learning/perfecting any skill. Time, commitment, and repetition are required. A few examples of this are Shifu/Po meditating to have inner peace, Tigress hitting trees to harden her fists, and the Furious Five training all day in the Training Hall to stay in shape. Also, at the cost of investing in one skill, you're not investing as much in other skills. You're choosing one path of life over countless others.
Let's talk about Po/Furious Five and the villagers in the Valley of Peace. While Po/Furious Five specialize in fighting, the villagers specialize in skills like farming, cooking, and trading. Both groups can dabble in each other's skill, but the specialist would be more knowledgeable/efficient on what they specialize and the society would benefit more from specialization. Po/Furious Five protects the villagers in exchange for the services provided by villagers. It wouldn't be good for the Valley of Peace if Po/Furious Five were average fighters and if the villagers didn't grow enough food. So this specialized relationship will stay the same until baddies stop attacking the Valley of Peace.
Another weakness of Kung Fu is that it doesn't turn you into an omniscient and omnipotent god.
Oogway lived for thousands of years, yet he still died from old age. Tai Lung and Thundering Rhino were great fighters, yet they lost/died because they didn't know the capabilities of their opponent. Po/Furious Five were still hurt by the indirect explosive forces created by Shen's cannonballs.