Asm.js is just a mangled subset of JavaScript.

Every Emscripten demo I've seen so far has either not worked, or been horribly slow. As far as I'm concerned, it really isn't a viable option at this point for anything remotely serious. It may be an interesting experimental idea, but it still needs a massive amount of work before it's seriously usable.

In the end, they're merely an abuse of JavaScript, rather than providing proper support like NDK provides for C and C++ under Android, for instance.