Is there any? I think most modern compiler share the same codegen backend.

You don't get the benefit of the frontend when you transpile.

JIT-focused .NET is one of the ecosystems disparate from AOT-focused LLVM. While there's a bit more cross pollination now, at the time the C# to C++ transpiler was authored, there was much less so. I've taken a stab at porting mono to a new platform at a similar time period - it was rather nontrivial (I ran out of time and thus failed.) Worse still, many of Unity's targets (e.g. iOS, XB1, ...) explicitly ban JIT technology in the name of security, which wasn't something I had to deal with.

A MSIL bytecode -> C++ translator might be pretty quick and dirty... yet effective, gives you AOT compilation, avoids the need to explicitly target every architecture by hand. For all it's faults, it's not too terribly hard to figure out how to compile C++ for a given platform, typically, generally requiring exactly zero reverse engineering.