Unfortunately, most videos on YouTube along the lines "I built a rocket motor" are made by clueless people who got lucky not to hurt themselves, others, and not burn to the ground their or other people's property in the process. They should be only used as an example how to not make rocket motors. Not saying every single one, but vast majority of those.

They are becoming a scourge of the hobby. Similar how quadcopter idiots did serious harm to model airplanes hobby recently.