I would say that one restriction with the type of macros that elixir has, irrespective of whether we call it homoiconic or not, is that they can only manipulate existing forms in the AST, they can't define new syntactic forms. In elixir this means that you basically work with function calls. There is syntactic support for making the function calls look less like function calls but the macros you define are basically function calls.

In Lisp you are free to create completely new syntactic forms. Whether this is a feature of the homoiconicity of Lisp or of Lisp itself is another question as the Lisp syntax is very simple and everything basically has the same structure anyway. Some people say Lisp has no syntax.

Trying to build the AST that directly in elixir is quite difficult as the AST is not trivial even if it relatively simple and regular. Quote and unquote in macros do a lot of the work for you. At one level you could do something similar for Erlang but no one has actually gone all the way with it. Erlang uses parse transforms for that. They are more powerful but much more complex to use.