I've never had any problems at any gait. I think they're much safer than shoes in most situations, especially slick rock. Soft shale is probably the only one where I'd rather have shoes...but certainly not enough to shoe my horses!
I've had boots come off a couple times (I was in a rush and didn't adjust them properly, but once because the cable broke) and although it's always happened at a walk, it hasn't been a big deal. Thinking about it now, considering what I ask them to cross on the trail, the boots are pretty soft and flexible...not really a big deal

The only ones I haven't had come off are my Epics, but I'm sure they could if the cable broke. I was worried about it too, but I have a couple thousand miles with boots on and no problems yet.