Structural damage to wheels generally occurs because of impacts that are hard enough to negate the protection of the tires. Any wheel can and will bend if it gets hit hard enough, however there are some significant differences among wheels as to what kind of an impact it takes to bend it, and how the metal will react to being bent.
Aluminum wheels come in a wide range of alloy compositions, from very soft and pliable to very hard and brittle. Less nickel added to the aluminum gives an alloy which is lighter and softer. Soft alloys bend easily, but do not tend to crack except under extremely hard or sharp impacts. More nickel creates an alloy that is heavier and quite a bit harder and more brittle. Such wheels do not bend as easily, but will crack under impacts that will only bend softer wheels. European, particularly German wheels tend to be much softer, while American and Japanese wheels tend to be more brittle. Steel wheels are extremely difficult to bend at all, and will almost never crack.
When a wheel is subjected to a strong enough impact, the metal bends. It's much easier for a wheel to bend on the inside away from the support of the spokes. The problem with this is that the bend can be nearly impossible to see when the wheel is on the car, and so the first indication of a bent wheel is usually the vibration that it causes.
It is also possible to bend a wheel on the outboard, or cosmetic face. Such bends are generally more difficult to straighten, and damage to this area has a much greater chance of being fatal. Deep-dish wheels are particularly vulnerable to front face damage because the spokes are set back away from the outer edge. A good impact can potentially fold the dish against a spoke and destroy the wheel.