Vs. Light Scattering
USAXS and Light Scattering investigate the same length scales, but exploit the very different characteristics of visible light and x-rays, to investigate very different materials. Below are some examples of where X-ray scattering should be used instead of Light Scattering
Issues with Light Scattering: Opacity and multiple scattering
Consider first a solid sample like a polycrystalline material or powder that has been sintered. In most cases, light cannot penetrate the sample because of optical opacity, so we need to use X-rays. And even in samples, which have optical transparency, x-rays may be preferred as light scattering is subject to multiple scattering effects. This is especially true in materials with voids or many domains.
Both these challenges are not issues for USAXS, where the radiation is indeed penetrating and the interaction of the radiation with the material is still comfortably within the limit of single scattering.
USAXS: A-priori structure calculation
Furthermore, in light scattering, the scattering contrast is subject to local environments and bonding characteristics that are not known a priori, while for x-rays the scattering strength of the material is known from first principles, making calculation of structure based on absolute scattering a more feasible task.
USAXS: More than just size
Finally, for loose powder samples, it is just easier to prepare samples for USAXS than for light scattering. Techniques have been developed for light scattering, which require making a slurry and pumping this fluidized mixture through the light scattering cell. This is a fast measurement, and can be modeled to determine an average particle size. USAXS, however is not limited to particle size, but can also give you the size distribution, as well as the more subtle features of the material, including surface roughness and even shape.