I don't think any of that is the reason many TMA folks don't generate as much power. It's the root of the back leg. I think (and this is supposition) this comes from earlier, when they all (including Karate, which now sometimes doesn't) recognized takedowns, and probably practiced them. So they held weight back a bit for stability. Boxing long ago stopped worrying with that, and brings power all the way up that back leg on power strikes, shifting more weight into it. That's less risky in boxing, and we see a bit less of it in MMA, though they often use the boxing mechanics (so we see the shift at the leg) because it's more mobile.