I've done an online class before and I enjoyed it. Had homework and everything. The only thing is that you have to be super dedicated. When given an assignment to go train a technique or do an exercise then you have to do it. You have to be able to put aside any dislike of not being able to do something well as the only goal is to do it and to show it on video for the instructor and other students to look at it. It's uncomfortable at first but you have to brush that aside. I took an online course for kung fu. I enjoyed. But I think I enjoyed it because I already had experience in the system and the instructor knew that he was teaching people who already trained the system. I don't think I would have enjoyed it if I was a beginner.
I think the instructor has to be really aware of the extra details that may affect how it's taught. Watching exercise videos on tape works and it's successful. We have decades of proof that it can be done. So I think it's a good thing if your goal is to stay in shape and learn martial arts as a way to stay fit and as a way train in a martial arts system. However, if you are training martial arts because you want to use it, then there is going to be some limitations.
After many years of talking about martial arts and hearing different views. I have come to this conclusion
Online martial arts training is good for learning a martial arts system and some basic functions. This is a changed mindset for me because not everyone takes martial arts so they can learn to fight with it. There are some attack and defense basics that can be taught online and people can do those fairly well. Front kick+ front kick variations can be taught online. And you can get a lot of mileage out of it. Forms can be taught online. Applications training can be taught to a certain point, there's a lot of key stuff that comes with applications training that I would recommend doing offline with the instructor. There are some things you can do with friends and you'll be fine. There are other things that you'll learn quicker and better if you have someone experience perform the technique on you, so that you can understand better what you should be trying to accomplish with a technique.
If training in person ranks 10 in quality then training online is about a 6 or 7. I think most people who train at a school in person are around 6 or 7. They are either training to stay in shape, training to do forms competition, or training just to say they have a black belt in a martial arts. They aren't really training application for use. The application that they train is more like application knowledge and not application function.
For me personally I could live with a 6 or 7 for a new martial arts, but that's only because Jow Ga kung fu is my applications martial arts. It's the one I fight with. I could also see myself take an online martial arts class as a compromise. For example, If I never took Jow Ga kung fu before. I would take an online class as a beginner, with the mindset that hopefully in the future I'll be able to take an in person class. In this scenario. I could start learning some Jow Ga kung fu or spend years not learning a little of it, not being exposed to it, and not training it. Or I can just not take it and wait for a school to come in my area which only happened once in the 17 years, I've been living in my location.
Is having a little better than having none? The answer may be different depending on your goals. If you are going to actually use a martial arts to fight with then at the minimum, you'll need a sparring partner and some in person instruction for certain techniques. If you happy with what most people get out of martial arts at that 6 and 7. Then an online class may be good for you. Especially if it's a live class. The online class that I took wasn't a live online class. It didn't like that part.