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Actually, Jimmy's analogies are good here, and they're being held back by your lack of understanding of engineering.
For example, while erosion failure is a source of long-term damage to a dam, truly catastrophic failures are caused by water pressure. This is why many dams cannot be used to prevent flooding altogether. The river's flow through the spillway can be blocked and the water will accumulate, until at some point the force placed on the dam by the water exceeds the dam's structural strength, something warps or bends or cracks, and a dam failure results, much to the displeasure of anyone downstream. It's not "well, we're worried that the rushing water will wear away the dam," it's "if we don't open the gates the whole dam will go and you'll get the entire lake at once."
You can't make an invulnerable dam out of stainless steel and alloys. It would be stronger, to be sure, and certainly more corrosion-resistant. However, it too will have a finite amount of water that it can hold back, and that finite amount isn't necessarily the amount of water it takes to overtop the dam; if the lake behind the dam is more full than your design specification calls for, the dam will fail.
I'm hesitant to use Reinforce as a source of analogy. She's not a mage, or even very like a mage; furthermore, she's damaged and not functioning properly to begin with. It's very hard to extrapolate from her failure mode to the "normal" functioning of a Mid mage.
It's also pointless to talk about beam and shield "density" like it was something we could measure. Bleh. We can discuss their behavior as observed, but saying "Nanoha should shoot super-time-and-space-compressed Divine Busters" isn't much different from saying "Goku should shoot ten Kamehamehas at once instead of just one!"
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