How is that worse than carrying a passenger?
A passenger, when acting as a co-rider, may actually offer some enhancement to rider control.
Still, in the Owner's Handbook, Triumph stipulate (Accessories and Loading) - in addition to the many warnings of potential hazards of carrying passengers - some specific warnings about accessory payloads.
"The presence of accessories and/or payload will cause changes in the stability and handling of the motorcycle."
"Failure to allow for changes in motorcycle stability may lead to loss of control or an accident."
Carrying of objects in excess of 5 kg in weight, that...extend beyond the rear...of the motorcycle may lead to loss of motorcycle control...