In my opinion all domain-specific languages (DSL) are underestimated, because their use has a very narrow niche. Thus- it is used by a small segment of targeted developers and is mostly unknown to mainstream developers and/or to the general public.
One example is GLSL. It is mainly used for programming different types of shaders (vertex, fragment, geometry, compute,..etc) which targets OpenGL graphics library for processing data pipeline on video card. The basic reason why GLSL is underestimated in PC development is that video pipeline processing is mostly important in PC video games and these are mostly produced for Windows operating system, which mostly uses other proprietary graphics library DirectX. Which in turn uses it's own proprietary HLSL shading language for video shaders programming. Some Windows games also supports OpenGL library, but these are minority. So GLSL and OpenGL targets mostly Linux operating system's video applications. Of course HLSL is also a type of DSL, thus is also an underrepresented programming language. However I wanted to expose GLSL more, because it's domain is yet more narrow in PC development than HLSL, as market share of computer video games shows.