The level of "convoluted" depends on which level you want to look at. The asm.js spec is dead-simple, almost purely "mathematical" if you like. But if you require that whatever GCC/clang would have outputted for a specific C file is the exact same binary code that gets executed in the browser, then yes - you are not getting that. But then again, asm.js just - on a high level - specifies the primitive low-level constructs that C/C++ programs employ. Mapping them to instructions is a pretty straightforward job, but of course you are not getting 100% exact same results on every browser - just like you are probably not getting 100% exact same results from clang and GCC for the same C source code.
It's more that I'd like browsers not doing an absurd amount of unnecessary work and overhead. Leaving aside opinions on JavaScript as a scripting language for humans, it's not a sensible intermediate format for compiled binaries. It happens to work as a (very impressive) hack.
Even if it doesn't appear to be for those who don't consider the whole context it's a reasonably effective intermediate format. The current success of it among the vendors proves that.
