I've never been good at coloring my own hair. I'm left-handed and I always end up with a spot in the back towards the right that I overlook/forget/can't figure out how to get to it. My hair is thick and curly so hitting every inch of it is difficult, or always was for me. I've also never found an OTC product that looked as high quality as what my colorist uses, and I suspect henna is the same. The chick in the video is right that any gray using henna will be tinted red and not brown. She looks young, but she'll figure that out as she ages and gets more and more gray, henna will stop working for her if she wants to stay a brunette.
The woman who cuts my hair uses great cruelty free products and does a great job. She also refills my shampoo bottle. I get a color once every 5-6 weeks so I never have to think about it between appointments.