What we in fact see is that the object that "barely travels any distance at all" is the spent shell casing. This is completely fine as the aim is not to magically embed the spent shell casing into the target. That is what the projectile part is for. The projectile is likely to have whizzed off as expected, albeit not with great accuracy.
As for the general usefulness of plastic firearms, even if they can only fire a few shots, there are clear advantages.
1. You can obtain a firearm without it being registered to you or exposing yourself to criminal firearms dealers/police sting operations.
2. They are less detectable.
3. You can melt and/or burn the murder weapon with ease.
The tone of the video is a bit odd. It's comes across like a video trying to convince kids not to play with fireworks. It's not as if we all have loads of ammunition laying about here in the UK just waiting for a 3D Printed gun to come along so we can finally have some fun. Making something that can fire a bullet (at least here in the UK) is not the main obstacle to a working firearm. The main obstacle here is obtaining the ammunition.