Are you using OpenGL ES2 or ES3? I found the ES3 build (WebGL2) to not work as well on mobile devices, or even some desktop setups. It could be an optimization for mobile, as there are settings in the Project Settings that reduce quality or effects when running on a mobile device or on the web. It could also be the environment. I found that it's best to create a WorldEnvironment node and put a panorama texture or a custom sky, as the default one is really only for testing and doesn't really look good.