Anything written in ancient Greek by someone other than a native speaker of ancient Greek, to some degree or other, is inherently under a cloud of suspicion.
The ancient Greek language has been studied very extensively and in great depth, particularly over the last two hundred or so (but discoveries about syntax and vocabulary are still being made). And in the not-so-recent past, youngsters (especially in England and Germany) started Greek very early and were rigorously trained to read and write ancient Greek, to the point where they could achieve a very high level of competence. However, there is necessarily some suspicion that a non-native speaker might miss some subtlety of the ancient language.
Take almost any ancient Greek author, and you will find divergences of interpretation over specific passages even among very highly trained specialists. And there are ordinarily textual issues that often make it difficult to know whether we're dealing with authentic and idiomatic ancient Greek or a corrupt text. Also, the Greek language, like all languages, has been in a constant state of change over the course of its history. There are even differences between 5th century BCE and 4th century BCE Attic usages, and later ancient authors attempting to write pure Attic Greek of this period aren't themselves entirely reliable.
I'm not sure that that answers your question, but those are some of the difficulties that stand in the way of modern pedagogues attempting to replicate ancient Greek with perfect fidelity. Still, for learning purposes in relatively early stages, made-up Greek using a limited vocabulary is undoubtedly far more useful than harmful, and somewhat simplified but real ancient Greek can be helpful, too.