The gotcha with C# structs is that they're passed by value. If I pass a struct, and then the callee mutates one of its fields, this isn't reflected in the calle

The gotcha with C# structs is that they're passed by value. If I pass a struct, and then the callee mutates one of its fields, this isn't reflected in the caller. JavaScript values other than primitives do not work this way. If your codebase is entirely C# and you're just compiling it to JS so that it can run in browsers, then the compiler can fix this by inserting copies where needed. (The Turing-completeness of JavaScript implies that something like this must be possible.) However, if you want to be able to freely call back and forth between C# code and JavaScript code that doesn't know about this calling convention, while maintaining the full semantics of both languages, then you are going to run into problems. That's what I'm claiming is hard. There's no conceptual barrier to reflection; the problem is just that the required type information might be a lot of bytes, and it might be hard to tell for sure whether it's actually used at runtime. You might also have to ship a lot of support code that emulates enough of the CLR to make these operations work. So there'd be a strong temptation to just not support these features, which then would cause dependencies to break unpredictably.