No idea how learning C++ is "highly discouraged" lol Before addressing your concerns I would say comparing to C# and C++ the biggest problem Nim has is that it's a smaller language. There are less and less mature libraries and frameworks, you will get less support for your problems, etc etc. This has not really been a problem for me and how I've been using the language, but I'm guessing it would be most people's biggest obstacle in using Nim. (In fact this is the reason for those timed out benchmarks, a pure Nim library for bigints is being used that isn't as fast or mature as other implementations)
If you are truly meticulous about performance I don't see why you would choose C# over Nim. Nim allows you to generate basically any C code with abstractions to make it palatable and I don't think there's much debate in how fast C is. But even in the pure language, reference types aren't the default, there's no hidden dynamic dispatch, you can tune runtime checks and memory management. Even if some benchmarks show bad results they will not always take full advantage of the language. There are also additional upsides for game development. The compile times are good, you can compile to JS as well as C and C++, these backends also make the language very portable. But the biggest thing that ties everything together is the metaprogramming, for which Nim has basically no competition. Since it's not my job to sell Nim I won't continue much and others can look into these themselves
