> Who guarantees that if you combine `my-dsl-1-macro` with `my-dsl-2-macro` or simply with general purpose code, that they don't interfere in odd ways?

Nobody does. But that's where the power lies. You can't have unrestricted power with restricted features. Lisp gives you unrestricted power. It's up to you to use it correctly, for w/e definition of correct your context makes sense.