Language doesn't have much to do with rendering unless you're talking about CUDA or a direct GPU code. C++ and C# both interface with the same libraries. I program in both, but C++ does me zero good for my Unity needs, but if it is a C++ DLL I could interface with it that way. I'm not a fan of that though unless the DLL provides source code. I code in C++, C and many other languages. I am language agnostic other than I am using Unity which means C++ is not a good choice there when it comes to accessibility by the common person within the engine. Yet C++ vs C# actually has zero to do with rendering quality. Overall speed of code for C++ is generally faster though as there is no intermediate byte code based language, yet that doesn't matter in this case as Unity doesn't use C++ and that is the engine that I (and the majority of indies) am using at the moment. If I were using Unreal then I'd be wanting it in C++. Yet once I see some examples of how it is done in either language I could likely convert it to either type easily. :)