A breakup node that is outside the audible band will impact the audible band by IM distortion. Using red book, most metal domes will have a breakup above the FR range. Analog and true hires recordings can have energy that may cause the driver distort. Typically, these ranges are lower in level and thus the IM distortion will be accordingly lower. The use of exotic materials is one way to push the breakup out the range of most content. Another is engineering design, which can take a material quite a far with shape, variable thickness and stiffness, and other design features.
Soft domes will breakup quite a bit earlier, and the exact degree will vary based on the doping materials. The distortion will be more benign due to the internal damping properties. Other distortions are more present with soft domes as compared to other more rigid domes, but the internal damping does help to keep the issues generally benign. You don't have this kind of forgiveness in most rigid domes.
The exotic material choices is an effort to get a more ideal driver. Still, its only a single piece, or a single chapter, in describing a drivers performance.
