My opinion is that YouTube is a good example of a technology that has harmed education.
The "good" use case is for when you need to show a film of something you can't directly do yourself, or by definition cannot be done without access to video. In science, that might be a slow motion film of crystal growth or a clip of a rocket launch, or blowing up a lump of caesium in a bath tub. In English, it might be a great actor reading a sonnet. In geography, it might be the waves acting on a cliff. You get the point, and it makes sense.
Unfortunately, what it *became* is teachers offloading their explanation. You open YouTube, type in the topic, and presto you get a video. At best, you get BBC Bitesize. At worst, and ordinarily, you get some American cartoon with nice graphics and a good jingle in the background.
Even the best option isn't great. These videos are almost always completely unsuitable. They move too fast, include vocabulary your students don't know, aren't tied to your curriculum, and rarely adhere to the basic principles of how to construct an explanation.
I see this a LOT in my journeys around different schools. And I always test my theories. I ask students basic questions about the videos and the content, and invariably they just don't understand it (or worse, actively pick up misconceptions).
One of the sadder things here is that the construction of an explanation is the beating heart of the lesson, and it should be a thing of joy and excitement for a teacher to sit and really figure out how they are going to communicate their subject. After watching a lesson with a video, I'll almost always ask the teacher "do you really not think you could have done a better job yourself?" It's a punchy thing to say, but I've never had anyone reply "no" yet.
Can YouTube be used well? Sure. Is it? No. Can we all do better? Hells yeah.