The issue with preprepared demos is that you can carefully curate the scenario and make choices that you know are likely to show the best outcome, out of a wide

The issue with preprepared demos is that you can carefully curate the scenario and make choices that you know are likely to show the best outcome, out of a wide range of possibilities. If you know your model (or VC product demo etc) performs poorly under certain conditions, you simply avoid them. This is a reason to be somewhat skeptical about demos.