"Code review" in the most literal sense is already allowed, since
lilypond questions occur here fairly frequently. But for us it's less "review" and more "how do I do this?" or "what am I doing wrong?"
As for your two examples, they both seem too opinion-based to succeed here. Even though we do allow some subjective questions (see the "What Not to Ask" page), your two examples still seem to stretch the limits. Difficulty is subjective, so there is no right answer for your Beethoven question. And your Chopin-style question is a little unclear to me, but it still seems completely opinion-based if we're to "review" the piece and offer suggestions (which, again, are completely subjective).