Efficiency and ease of debugging? Just because it's possible to transpile to JS doesn't mean the resulting code will be fast/responsive or easy to debug. Which in turn means it won't be used for significant projects.
Debugging will use SourceMaps which already exist today. As for efficiency, I haven't read anything that suggests WebAssembly will be more efficient than asm.js, why would it be?
"Why: asm.js is great, but once engines optimize for it, the parser becomes the hot spot — very hot on mobile devices. Transport compression is required and saves bandwidth, but decompression before parsing hurts."
