the times I have ridden over really really really bad roads, especially dirt stuff where I'm all over the place, I've immediately thought that using the Ortlieb isn't a good idea, too rough on it---and lets face it, strap on bar rolls etc (of which there are a ton of options) are going to just plain work better.
Thats why all this stuff is popular and suited to real offroad and rough riding.

that said, I've been pretty impressed with how well teh Ortlieb mount handled it all--although I am not one of those people who are "breakers of stuff" because they cluelessly beat the crap out of stuff and then wonder why-slash-complain when X breaks or wears out. I ride accordingly and slow down in response to how much my handlebag is getting shaken to kingdom come, and clearly there are riding situations where a roll bag would make more sense, or a small front rack with stuff strapped to it etc.