Motorcycle hitch carriers

Many articles claim hitch carriers will damage a vehical's frame or tow hitch.
I have been using one for years on my Ford Windstar to haul 250cc and smaller motorcycles. The carrier is rated at 400lb. The hitch and Windstar are rated at 350lb tongue weight.
Over the years I have carried motorcycles thousands of miles like this, and never had a problem.
Have I just been lucky? 
Does anyone here know of actual cases where these carriers damaged a vehical? (as long as carrier and tongue specs were not exceeded.)