One problem I see is that it helps creates more way to do basic things like declaring a class or a function. (There are already a number of different utility libraries for OO in JS).

Now imagine if every JS codebase use a slightly different convention for class/function declaration with slightly different behaviours. It can become a nightmare to understand even the most simple function definition, just because you don't this particular "def" has some side-effects.

Now, I agree that in some cases, macros can be useful and reduce the complexity of the program, but declaring classes and functions certainly isn't the best use case for them and it's a bit sad those are the two first examples on the page.