There are certainly a lot of variables that contribute to this sort of event. A rider not bracing himself or herself before braking could very well be a significant factor. I have two bikes with Shimano hydraulic disc brakes, and they stop well. But I don't ever feel at risk of going over the bar. I'd have to clamp really hard, and really fast, on that front brake to be at risk of that. But when I do that, I find myself instinctively bracing against the bar and really pushing back into the seat with my butt (to move weight downward and rearward). If I were standing up on the pedals and sort of hunched over the front bar, maybe I could coax myself over with a firm squeeze on the front brake.

I've never ridden a mechanical disc brake system that would put me over the bar. I also own a few bikes with pretty effective rim brakes, and I don't consider that much of a risk there, either. Again, I could probably get that to happen if I set myself up for it (weight high, no bracing, etc.), but I don't think I could get it to happen with generally-accepted "good" technique.