Interesting photo. I wonder how it compares in terms of stiffness, strength and weight to the typical twin spar aluminium sportsbike frame?
IIRC, I read something to the effect that it's a "flex frame", so it wouldn't be as stiff as a tube frame. The flex should give it some strength, in that it won't be as prone to snapping, as if it were more rigid. and, it's a steel frame, so it's heavy, but it's also fairly "small", so it's not excessively heavy. I would have to guess comparable, or maybe a little heavier.
Most people say that the 919 is "too" heavy, for a naked bike, so I wouldn't doubt if the frame is contributing to that.
All in all, for the type of riding I do,.. it's more than strong enough. I think if you were a serious hooligan, there might be problems long term, but then, I'm not an engineer, so I may be out to lunch.