Explanation
The universal failure is order. Most demos spend four seconds establishing what the product is before showing it do anything, and nobody is still watching by then. Reversing it, so the trick happens first and the context arrives second, is the single change that fixes most underperforming demos.
The other constraint is that short-form has room for one claim. A demo covering four features is four videos that were stapled together, and it will underperform all four of them.
The common mistake
Opening on a logo or a brand card. It signals advertisement in the first frame, on surfaces where being read as an advertisement is the most expensive thing that can happen to a video.