It could depend on the intensity of the blast, the stronger the setting, the less of a mess left behind. I mean, if the guyver can lock on to a target's sonic frequency, they're supposed to be able to shake any unshielded target apart. However, if a guyver doesn't have time to find the specific frequency or doesn't fire a full sonic blast, it makes sense that they'd just blow the target apart rather than disintegrate it.