[klibertp]

IIRC, the criticism is of syntax-rules and syntax-case kind of macros in Scheme; I don't recall (happy to be proved wrong) LoL directly mentioning syntax-parse that's specific to Racket. syntax-parse is no less powerful than defmacro; it's just (much) more structured and featureful. It's a difference between hand-rolling a recursive-descent parser and using a (really robust) PEG library.

Racket's define-syntax/syntax-parse has escape hatches into procedural generation (through which you can easily break the hygiene), and the equivalent of CL's &environment contains more information and allows more operations in Racket. Racket also supports reader extensions but often doesn't need them because it has a macro-based API for module definitions (a feature completely absent from, or even alien to, Common Lisp; OTOH, Elixir borrows some of Racket's ideas in this space).

For a practical example, Coalton and Typed Racket implementations show that both systems are capable of large-scale language extensions. I never studied their respective codebases, so I can't say anything about relative ease of introducing those extensions, but they are possible in both cases, at least.