Are 60-foot articulated buses particularly risky / dangerous at highway speeds?

I'm in the Vancouver BC area, and our transit agency tends to employ 60' articulated buses on high-capacity urban routes, while double deckers are typically reserved for use on longer-distance / regional express routes that run on highways.

Is this just an arbitrary operational decision, or are 60-foot articulated buses inherently less safe / easy to handle at ~100 km/h (~60 mph)?