The primary downside of online training in my experience is much the same as the issue of trying to train by correspondence course, a lack of feedback a good bit of the time. Theoretically, they can view your technique over videoconferencing software, but in general, it's an inferior method because they can only really see from one angle, and only if your movements keep you close to the screen, plus if they have more than one student, they can usually only view one feed at a time.
It is, in my opinion, better than nothing, particularly if you have difficulty self-motivating (I know I do), and it is much more accessible if you don't have classes in your area, can't leave the house for classes (I'm sometimes in that situation myself due to a new child requiring a lot of my time, with me sometimes only being able to carve out enough time for the class itself), or you can't afford the in-person classes (a lot of teachers are offering discounted Zoom classes since they don't have to foot the bill for studio space or insurance).
For use in supplementation, I'd say to use them on those days you can't attend in person, for so long as they're offering a benefit to you. It's not perfect, but it's better than nothing, and having an actual teacher can push you outside of your comfort zone by suggesting novel movements or combinations. Plus, well, it can be handy if your own personal practice might tend towards "do 10-15 minutes, get bored, do something else" if not in a class that's being run.