Mono is a well designed, very portable codebase. The only platform I can imagine having trouble _is_ the browser, but I would bet that you could compile mono with emscripten quite easily. Also, there's http://www.jsil.org/
There are also issues with Mono on some consoles and mobile devices, due to how linking works as well as some non-technical (legal) restrictions the platform owners place.
From talking to Mono devs, definitely Mono can be ported to new platforms, at least if there are no license problems. But even then, it can be quite a lot of work. il2cpp avoids all those issues, it emits C++ which every significant platform can handle very well.
From talking to Mono devs, definitely Mono can be ported to new platforms, at least if there are no license problems. But even then, it can be quite a lot of work. il2cpp avoids all those issues, it emits C++ which every significant platform can handle very well.