The more experience I gain, the more I realize mirrors are deceiving. We have mirrors on the front and back wall where I train. We start practically every class with basics thrown in the air. I watch myself often enough in the mirrors during basics and kata. I think I look pretty good; not perfect by any means, but good enough. Well, except kicks above my waist
I was preparing for a tournament a few months back that I had to back out of due to an injury (not training related). I had an instructor video me performing the kata. I saw quite a few things I didn’t see in the mirrors. I’ve videoed myself doing basics and hitting a bag. I saw quite a few things that I didn’t see in the mirrors.
And no, it wasn’t because the camera was at a different angle. The kata and basics were straight on, like the mirrors would’ve been.
Mirrors typically show me start and stop points, and the majority of the actual motion is lost to me. Watching it on video showed me how much I didn’t see. And when watching in the mirrors, I’m only seeing/focusing on one aspect/body part move. In video I see my whole body move in real time. I’m seeing what everyone else sees, and it’s different from what I see in the mirrors and different from what I’m “seeing” in my head. It’s kind of like singing - you may hear the pitch and annunciation correctly while you’re signing it, but if you record it and play it back, it can be quite different. A lot of people think they’re a good singer until they hear their recorded voice; a lot of people think they’re doing a technique nearly perfect until they see themselves on video, even if they’ve done it in front of mirrors a million times.
Another way to look at it is like watching a recorded sparring match. You see a ton of things on video that you didn’t see during the match because you were focused on very specific things, thereby giving you tunnel vision. Performing solo stuff in front of mirrors will give you tunnel vision too, but to a lesser extent.
As far as using mirrors vs hitting bags, and vs video that I’ve brought into the equation... you need a balance of all 3. Mirrors, bags, and video each give you a different type of feedback. IMO the mirrors probably give the most limited feedback, but they’re still helpful. If you’ve never recorded yourself, you really should give it a try. You’ll most likely be surprised by what you see.