Some random thoughts...
Video instruction can be quite valuable. I can honestly say that instructional videos played a major part in helping me earn my BJJ black belt.
Just watching videos isn't going to get you anywhere. By the same token, if you just sat and watched live classes at the dojo without ever participating you wouldn't develop any skills either. You have to put in the work either way.
Practicing what you see in the videos is necessary, but some sort of feedback to make corrections is also important. If you don't have an instructor handy, then that feedback will have to come from your training/sparring partners. The better training partners you have, the better feedback you will get.
Some people are visual learners and will be able to get more from video instruction than those who are not.
The more experience you have, the more you will be able to learn from video. The more specific that experience is to the material being covered, the more you will be able to learn. In BJJ I have enough experience that sometimes I can watch a video, then go in to the gym next day and be tapping people out in sparring with what I've learned from that video. In contrast, I'm a beginner in Wing Tsun, so I'm able to absorb a lot less from watching a WT video and need more hands-on guidance to fully understand what I'm seeing. I'm
highly skeptical that a novice will have the ability to develop usable martial arts skills primarily through video instruction.
There is a huge range in the quality of video lessons from an instructional standpoint. Many top notch practitioners are terrible at structuring a video lesson so that you can pull anything usable from it. Others are brilliant at organizing the material, making sure you can see everything clearly, explaining the details that make the techniques work, warning of pitfalls, and offering training and troubleshooting tips.
Another problem with books and videos is that no one actually trains. That's right, I'm saying it. You buy the video, you watch ten minutes of it while sitting on the couch eating cookies. You stand up and imitate a couple moves, you decide you've got it, and you never watch the video again. That's how it works.
I guess I'm no one then. I watch videos, drill the moves with a training partner, discover any parts which seem rough, go back to check the video to see if I can spot the problem, drill some more, try the moves in sparring, then go back to the video again to see if I can catch details I've been missing in application. I also use video to discover refinements on techniques I've been practicing for years. I also use videos to gain insights into concepts and principles which inform all aspects of my training.
I generally train about 10-12 hours per week on the mats*. On top of that, I would guess that I average a couple of hours per week studying video. (That's sort of a wild guesstimate. Some weeks I don't look at video at all. Other weeks I might watch 6+ hours worth.)
*(Or on wood floors or in the back yard or wherever.)