It doesn't matter what the assembly looks like, if the definition of language is well-defined. With macros this is not true, different 'macro' definitions probaly work in a lot of different ways.

It's true that different compilers and interpreters sometimes do slightly different things, but atleast there's a standard which should improve over time. I don't see that happening with macros.

Not sure how modularity is achieved with macros either.

Macros CAN be well-defined, but I think the cherry picking of macros mentioned in the article leads to a "language" that is ill-defined vs picking up CoffeeScript or LiveScript. The idea of implementing ES6 via macros is pretty attractive though.

Why can't macros be well defined?

They can be, but rarely are.

I think it's less about correctness of implementation and more about abuse of the interface. Not that I agree, but I think it's a completely valid criticism, in the same way that you could argue against operator overloading in OOP languages.

Ummm... yes? Sometimes you have to. I was looking into a bug about a year ago that was caused by the compiler I was using (Intel) wasn't properly saving the return address in some situations. This led to seemingly random crashes and thrashed memory.

So yeah, "correctness of implementation" is something to worry about, you just haven't had to (yet).