Is the Canon RF 100-400mm F5.6-8 IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 100mm f/5.6 test image. Observe the contrast and resolution of these results.
Is the Canon RF 100-400mm F5.6-8 IS USM Lens set to 100mm f/5.6 sharper than the Sigma 100-400mm f/5-6.3 DG OS HSM C Lens set to 100mm f/16.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.