Is YouTube a good way to learn a language? While I think it could be a good way to present a language course, I am not currently aware of any good language courses on YouTube. There are random videos with people giving language lessons but they are not structured well. A really good video course would be French in Action. It predates YouTube and is copyrighted so it is not likely to show up on YouTube. But to make a language learning course like that takes a lot of planning and production costs. We cannot expect such a fine product from an individual's home-made language lesson.
In fact, to make an excellent video course for language learning requires a genius. You have to present the language in a piece by piece offering and yet make it engaging. The audience should get a lot out of the current lesson and want to go right on to the next lesson. When the learner doesn't know that he is learning, then he can make real progress. And that's the beauty of film. Your whole attention can be taken up and you can SEE what is going on. When you see what is going on, then you know what is being talked about. Keep the phrases short in the beginning lessons and it will be obvious what the words mean.
Some languages are better for this than others. Why? Because in those languages, whole sentences are just one or two words long. And those are natural sentences, not baby sentences. For example, in Japanese, in order to say 'It is hot,' you need only one word, 'atsui.' That one word conveys all the meaning. No need for 'it.' What is 'it' anyway? And no need for 'is.' If you just said 'hot' in English, you would conceivably convey the meaning, but you would be criticized for using unnatural methods to teach.
One of the most critical aspects to presenting a language is the 'build-up' approach. The words and phrases that are introduced in one lesson need to keep showing up in the next and subsequent lessons. If not, they will just be forgotten. And I think this is where the current YouTube lessons are failing. As I said before, it takes planning. Not for each lesson but from one lesson to the next so that no new words are left behind.