The biggest advantage is non-technical: it has universal adoption in the browsers from early on, esp. on Apple devices. That was NOT an easy task to achieve. I do believe the previous attempt with asm.js was able to help there to ease the idea for the browser makers.

And technically it's quite well done. The only thing that is missing is thread support, but due to complexities I totally get why it wasn't done and it was a right call. There are workarounds and it will be added eventually, some forms of it exist already.