Shun knives, and Japanese knives in general, are made from harder steel than German or other Western knives. The advantage of the harder steel is that it holds an edge longer, so requires less frequent sharpening. The disadvantage of the harder steel is that it's brittle, so it's more prone to chipping than the softer (but stronger) steel used in German knives.
Japanese blades are also traditionally thinner than German blades, which can also make them more prone to chipping. However, most Japanese knives made for the Western market are not as thin as those sold in Japan, so this is less of an issue than the more brittle steel.
For this reason, Japanese knives should not be used for cutting through bones or other hard materials (like winter squash).