My observation is that any bed flex is the result of the under bed plate not being snugged upward tightly during the installation process. This allows some gapping under the bed between the bed and plate...which allows for bed flex.
I have zero issues with my 2011 Chevrolet and just returned from a 10,000 mile pull out west, and have towed a total of about 25,000 miles on this rig with a 15,000 lb fifth wheel. No loss of bolt torque or any other issues with the hitch.
I install my own hitches, and use a come along on a tripod to lift and pull the hitch plate into position before torquing bolts.
Having said all of that, it is my understanding that B&W has a new "in bed" stiffener for people with bed issues. Now, I also have a spray in bed liner so perhaps that helps.