Yes, both SunTouch and Nuheat products have been thoroughly tested and are UL listed for use in shower floors and seats. However, radiant floor heating is not approved to be installed in the walls of a shower. Installing radiant floor heat in any wall is a violation of housing and electric code.
Just keep in mind that mats and cables being installed in a shower cannot be repaired if they are cut during installation. Once the outer coasting of a wire has been cut, it is no longer waterproof. Even repair kits made by the manufacturers cannot restore the waterproofing.
Also, it is recommended that separate mats or cables be used to heat the shower floor and the bathroom floor. This is so the radiant heat in the bathroom floor will not need to be replaced if the shower is remodeled or repaired in the future.