S-mount lenses are much less expensive than c-mount lenses, so it would be fantastic if we could replace c-mount lenses with them.
A clearly visible difference between s-mount and c-mount lenses is the size.
To improve a lens, more lens elements generally have to be used. The maximum allowed footprint sets a natural upper limit, because an optimized lens can only be improved with additional lens elements.
Therefore, there must be objectives that can be built as a c-mount, but not as an s-mount.
Another class of lenses which “must” cause difficulties are lenses for very large sensors.
Due to the mechanical limit of the diameter being just 12mm, sensors of 1″ can hardly and sensors of 2/3″ can only be supported with difficulty, unless the sensors accept very large ChiefRay Angles (CRA).